In the autumn of 1999, Melvin Nelson Perry, a minor British art thief with dark hair and a penchant for Gary Glitter-style tinted glasses, strode into Copenhagen's Royal Library and asked to be directed to the map room. Even though Perry was carrying a folded newspaper under his arm, which was strictly against library rules, he passed through the first security check easily. The Royal Library had been inundated with foreign tourists since the opening the previous August of its Black Diamond extension, and the information officer probably assumed that, unlike the architecture students milling round the foyer, Perry was a genuine reader.

Founded in 1673 by Frederick III to house his private collection of rare books and manuscripts, the library had originally occupied a small building opposite Christiansborg Castle on Slotsholmen. But thanks to the progressive policies of successive Danish kings (the Royal Library was the first to throw open its doors to the public in 1793), the library's collections had grown rapidly and by the 90s it was in desperate need of more space. It was in an effort to uphold its proud tradition of continuous public access while protecting its stacks from light-fingered readers, such as Perry, that the library had commissioned the controversial extension - a trapezoid steel frame clad in Absolute black granite (hence its nickname, Black Diamond), the extension now dominates views along the waterfront. Inside, however, vast windows looking out on the Inderhavnen create a feeling of space and freedom.

This impression, however, is misleading, for following an embarrassing series of internal thefts in the 70s, the library had also insisted on installing CCTV and electronic-locking doors on every level. Nowhere was security tighter than in the map and rare-book reading rooms, where staff kept a careful eye over the prized collection of some 7,000 atlases and 270,000 maps - the largest in Scandinavia.

Perhaps Perry, who had previously been jailed in England for thefts from the British Library and Cambridge University Library and who sometimes spelled his name 'Perrie' (or when he was being really sneaky 'Perry Nelson'), thought the Royal Library would be an easy target, or perhaps this was merely meant to be a test run. Whatever, his approach was not very clever. Signing in under his correct name, Perry proceeded to the map room and, still carrying the newspaper under his arm, requested atlases by the early Dutch cartographers Abraham Ortelius and Jan Jansson, and the 17th-century British mapmaker John Speed.

Something about Perry immediately set the Danish map-room attendant on edge. It wasn't just the newspaper, which the attendant told him to remove, but Perry's accent. Although English was the attendant's second language, he found Perry's intonation 'vulgar'. Not only that but when he asked him to state the subject of his research, he wrote 'catography' - leaving out the first 'r'. Perry never returned to claim the atlases, probably because he sensed the attendant was on to him and didn't like the sight of all the CCTV cameras. Fifteen months later, on 29 January 2001, however, the library was not so lucky.

A former landscape gardener turned antiquarian book dealer from Leeds, Peter Joseph Bellwood also had previous convictions for theft. In 1996 Bellwood had been sentenced to four years in prison for razoring Victorian sporting prints and maps of the Holy Land from books at the British Library and libraries in Birmingham and Leeds. Nevertheless, like Perry, Bellwood made no attempt to disguise his appearance, signing in under his own name and proceeding directly to the Royal Library's map room, where he asked staff to help him locate maps by exactly the same master engravers. Unlike Perry, however, Bellwood was well-spoken and did not draw attention to himself by doing anything so stupid as carrying a newspaper or misspelling cartographer. Instead, dressed in a loose-fitting sweater, the bespectacled, white-haired Brit ingratiated himself with library staff by handing in a 500 Kroner (£46) note that he had supposedly found 'lying' on the floor. The result was that by 31 January Bellwood had succeeded in razoring three maps from the pages of old atlases and five from a rare 16th century Dutch travel book, the Itinerario. The maps included Petrus Plancius's lavish 1594 double-hemisphere world map, Orbis Terrarum, Abraham Ortelius's 1587 Typus Orbis Terrarum, and John Speed's 1627 map of North and South America.

In all, his haul was worth £100,000.

Two years later, these maps have still not been recovered, and Bellwood and Perry have disappeared. Although police suspect they may have been 'stealing to order' for a mysterious collector, so far no one has come forward with any names and it is equally possible that the maps were sold on by unscrupulous dealers (Speed's map, for instance, is particularly sought after by American collectors as it was the first to portray California as an island). But perhaps the most shocking aspect of the thefts was that no one had thought to inform the Royal Library earlier about Bellwood's and Perry's convictions, much less that they had served their time in Britain and were on the loose.

'If we'd known, we could have posted their names on our system; we could have made sure our staff were watching,' says Erland Kolding Nielsen, the library's director. 'What I would do to get my hands on them.'

Bellwood and Perry were not the first to exploit the open-access policy of public libraries. In the early 90s another map thief, Gilbert Bland, criss-crossed the United States, tearing some 250 maps from library atlases. Bland's infamous career came to an end in 1995 when a sharp-eyed librarian spotted him razoring a page from a book at the Peabody Library in Baltimore and was later immortalised in Miles Harvey's The Island of Lost Maps. In his book, Harvey tries to shed light on Bland's crimes by discovering as much as he can about his life, but he soon discovers that Bland is a cipher, a terra incognita. Instead, Harvey is reduced to mapping Bland's crimes. But in so doing he comes up against an unexpected obstacle - the refusal of many librarians, still embarrassed by the ease with which Bland has gained access to their collections, to talk about it.

Mapping Bellwood's and Perry's life and crimes presented similar problems. The first thing I discovered was that the Royal Library was not the first institution the pair had visited. Indeed, Bellwood - possibly with Perry's assistance - had almost certainly begun his razoring campaign in Wales in early 2000, before embarking on what amounted to a 'grand tour' of Scandinavia. In the course of their travels, the men - either working together or separately - succeeded in stealing more than 200 maps from libraries as far afield as Aberystwyth and Helsinki. To date only a handful of these maps has been recovered, and in the case of the National Library of Wales official embarrassment at the scale of Bellwood's crimes means that the library still refuses to discuss what is, or isn't, missing. It is precisely this culture of secrecy that, argue dealers concerned to clean up the trade, enabled Bellwood and Perry to get away with their crimes for so long and has hampered the effort to recover the missing items.

Like Harvey, I decided the best way to build up a picture of my map thieves was to revisit the scene of their crimes. Before travelling to Denmark to interview Nielsen, I studied a city map of Copenhagen, familiarising myself with the concentric layout of its streets and canals and running my finger along the route I would be taking to the library. The map helped me (it was while studying the map, for instance, that I realised the old imperial centre of Slothosen, where the library is located, was actually an island). But maps are about far more than latitude and longitude. They are also small representations of our world and as such have a special pull on our imaginations. One reason collectors lust after Speed's 1627 map of North America, for instance, is that they can no longer go there except on his map - cartographic assumptions have changed, California is no longer an island. Similarly, Antarctica, or Terra Australis as it was known in the 16th century, no longer occupies the bottom third of the globe, as it does on an old Ortelius or Plancius.

Then there is the sheer aesthetic pleasure of an antique map - the detail of the engraving, the vibrant use of colour, the elaborate pictorial borders, the harpies and sea demons lurking on the peripheraries. More than anything, an old map brings past perceptions of the world alive. As Graham Arader, one of the world's biggest map dealers, puts it on the website of his successful chain of North American galleries, the best maps represent 'moments of unprecedented innovation or beauty'. 'Even after three decades, I often find myself enthralled by a map... [that brings] the past into the present with vivid power.'

Arader should know. It was his practice in the 70s of breaking atlases and plate books and offering the individual maps for sale that helped fuel the inflation in prices. Indeed, were it not for Arader and other collectors' passion for geographical representations from the past, it is unlikely that Bellwood and Perry would have ever found their way to the Royal Library in Copenhagen.

Nielsen's first inkling that something was wrong came at 7pm on 31 January when his head of security, Jesper Jorgensen, received a call at home from the attendant in the rare books room. He had been growing steadily more suspicious of Bellwood for days. On the afternoon of 30 January, Bellwood had been upset to be told that the book he had ordered was stored off site and would not be available until the following morning. Then there was his 'sudden' discovery of the 500 Kroner note and the fact that he kept on getting up to leave the room. However, it wasn't until after Bellwood left the library and she saw a page sticking out of one of the books he had been studying that she realised various plates were missing. The book was Hugo van Linschoten's Itinerario. Published in Amsterdam in 1596 and featuring lavish engravings of the then known coasts of Africa, Arabia and South America, the Itinerario is widely considered one of the most important travel books ever printed. The Plancius world map, in which Asia is shown as an elaborately robed woman seated on a rhinoceros, for instance, commands £15,000 alone.

Jorgensen immediately alerted Nielsen and together they rewound the CCTV footage of Bellwood's visit. They couldn't believe their eyes. Slowing the frames, they watched aghast as Bellwood seemed to draw his hand along the binding of the book, then remove a page and place it under his seat. Minutes later he repeated the action, placing another page under the table. Then he stuffed the pages under his sweater and exited the library.

'We think he may have been wearing a special vest so he could smuggle the maps out without being detected,' says Jorgensen, amazed at the way Bellwood carried on in view of the CCTV cameras. That night, Jorgensen called the police and they suggested he spring a trap. But though Jorgensen waited all day, Bellwood never returned.

On 1 February, fearing Bellwood had fled the country, Jorgensen posted his photograph on the library's website, together with the details of his alleged crimes. In the normally sedate world of libraries, this was the curatorial equivalent of the detonation of a nuclear bomb. Within hours, news of Bellwood's audacious thefts had flashed round the world, prompting librarians to check their records to see if they had also been visited by the ripper.

'At this point,' explains Jorgensen, 'we believed only one thief was involved. Then I got a call from my colleagues at the Royal Library in the Hague.'

The Dutch informed Jorgensen that they had also discovered some maps - 55 in all - missing from various atlases the previous October. Not only that but at that very moment, the suspect - a British passport holder - was sitting in their map room. Although the library had informed Interpol and the Dutch antiquarian book trade about the thefts, until now they had kept the information confidential from other libraries. The man's name, they said, was 'Nelson Perry'. Jorgensen immediately went through the previous reader passes. That's when he discovered that Perry had visited Copenhagen in 1999 and requested exactly the same atlases as Bellwood.

For reasons that are still unclear, Perry fled the Royal Library in the Hague before he could be questioned. But in his hurry he left a notepad on the desk. In it, staff found a rough map of the streets around the Royal Library in Stockholm, together with the names and shelf references to atlases by five Dutch and English master cartographers and a note to 'check the editions'.

Jorgensen immediately faxed the pages to Stockholm and urged them to look at their collections. To their horror and dismay, the Swedes discovered that between August 2000 and January 2001 they had also received multiple visits from both a 'Nelson Perry' and a 'Peter Bellwood'. At least six atlases from a 1579 Ortelius to a 1635 Mercator had been mutilated and some 40 maps removed. Intriguingly, on two occasions Perry or Bellwood - there was some confusion among staff as to which reader was which - had also asked to see van Linschoten's Itinerario, but staff had grown suspicious and he had never signed for the book. There was another detail, too. Perry/Bellwood's last visit had come on 24 January 2001 - just five days before Bellwood turned up at the Royal Library. Despite the confusion over the men's identities, Swedish police had little doubt that Perry and Bellwood had been working together, though whether they had visited the library at the same time or separately was unclear.

By now, Perry must have been aware of the heightened security and the risks of continuing his tour. Nevertheless, on 21 February he turned up at the University Library in Helsinki and asked to see a series of maps from the collection of the 19th-century Finnish explorer Adolf Erik Nordenskiold. The first person to navigate the Northeast passage, Nordenskiold's collection of 600 atlases included a near complete set of Claudius Ptolemy's Geographia. First printed in Bologna in 1477, Ptolemy's atlas appeared in several editions before 1500 and contains one of the most sought-after maps of any period - his Mappa Mundi. This map can command anywhere from £80,000 to £120,000. Perry requested both the 1482 Ulm edition and a later 1486 edition, as well as a further two atlases by Jansson and Blaeu.

By now the Royal Library in the Hague had issued a warning about Perry, but due to an administrative oversight the news did not reach the staff in the Helsinki map room until after Perry had checked out. When, on 23 February, Perry boarded the budget airline Buzz and returned to England, it seemed as if the Finns had missed their chance. But on 16 March, police arrested Perry in London and initiated extradition proceedings. After six weeks of cat and mouse it looked as if the Scandinavians had finally got the upper hand.

It was around this time that news of Bellwood's and Perry's activities reached the desk of Tom Moulton, the British Library's head of security. Although Moulton had joined the library after its move to St Pancras, his staff had never forgotten the damage Bellwood had inflicted at the old Bloomsbury reading rooms in 1995 when he had made off with more than 1,000 plates worth £290,000.

'I asked a colleague who had been at the trial to check the picture on the Royal Library's website,' said Moulton. 'His reply was that it looked "horribly like Bellwood".'

Fearing the worst, Moulton ordered an immediate check of recent readers' tickets. To his relief, Bellwood did not appear to have applied for a new pass. Perry, however, had received a new reader's ticket just 10 days before. Interestingly, he had listed as his research subject 'Nordenskiold'.

Moulton quickly pulled up Perry's file. It was even bigger than Bellwood's. Over the years Perry had applied for at least four tickets either under his own name or aliases. On each occasion he had changed his appearance dramatically, most notably when as 'Thomas Cook' he had bushed up his hair and donned the Gary Glitter-style shades. Then in 1994, as Melvin West, he had visited the old North Library and made off with rare botanical plates from Captain Cook's voyage on the Endeavour - a theft for which, along with his visit to Cambridge University Library, he received a sentence of four years.

Moulton immediately showed the pictures to staff and established that although Perry had been up to the reading room, he had fled as soon as he had seen the CCTV cameras. Unfortunately, another British institution was not so fortunate.

Study a map of Wales and the first thing that strikes you is its geographical separation from England. Protected by the Cambrian Hills on one side and the Irish Sea on the other, Wales really is another country. Nowhere is this sense of isolation more acute than at the seaside town of Aberystwyth, home to the National Library of Wales. Perched high on a hill overlooking Cardigan Bay, the library's holdings include 22 different versions of the five variant editions of Ortelius's Theatrum Orbis Terrarum, as well as valuable 16th- and 17th-century editions of atlases by Speed, Blaeu, Jansson and Mercator. Or rather, they used to. For some time in early 2000, the library also had the misfortune to be visited by Bellwood. Bellwood didn't just sign in once but, encouraged by the absence of CCTV, again and again. As in Copenhagen a year later, he requested Speed's 1627 atlas, removing his New and Accurat Map of the World, as well as plates of each of the four continents, the British Isles and the most sought-after English counties. Next he turned his attention to the Orteliuses, razoring his valuable world map and plates of the four continents. Then he vandalised two valuable atlases by Gerard Mercator, dated 1619 and 1636. The result was that by the end of the year, Bellwood had succeeded in removing more than 100 plates from the library.

The penny dropped sometime in March 2001 when, alerted by the thefts at the Danish Royal Library, the National of Library of Wales decided to run a check. The discovery of the extent of Bellwood's larceny must have caused pandemonium. At a stroke - or to be more accurate, with several strokes - Bellwood had reduced the value of Wales's national holdings by £100,000. The library's priority, of course, was to get the maps back, but rather than publicise the thefts widely, as the Danes had done, the Welsh authorities decided to keep the news as quiet as possible. Although the Antiquarian Booksellers' Association (ABA) learnt of the thefts in March, it was four months before the library sent it a list of the stolen items and the association was able to forward the details to dealers in Britain and the US via its secure email directory. Many in the trade believe that delay was fatal.

'Dealers are far more likely to spot these maps on the market than the police,' explains Jonathan Potter, president of the ABA. 'The problem was that by the time we knew what was missing it is was too late. Most of the maps had probably already been sold on.'

At first the Welsh police respected the library's reticence, opting for a softly softly approach. Their first port of call was the village of Swillington, near Leeds, where Bellwood used to share a semi-detached house with his ex-wife Susan. But though Bellwood had briefly returned there after his release from prison in 1999, neighbours said he had not been seen for some time.

Not surprisingly, inquiries in the map trade drew a similar blank. 'You know that series Lovejoy?' asks Mark James, of Aberystwyth police, rhetorically. 'Well, let's just say that some dealers haven't been as helpful as they might have been.'

Finally, this past May, Scotland Yard decided to elevate Bellwood to its 'Most Wanted' list, posting his name alongside those of suspected murderers and rapists. But oddly, while the Yard was happy to hand out details about Bellwood, there was no mention of Perry. According to DS James, this is because police have no evidence to connect the two thieves. But in an email circulated to other librarians in March 2001 the National Library of Wales named both men as 'suspects', and to curators across Europe the connection is only too apparent.

'I believe there has to be a link between Bellwood and Perry because they were stealing the same kind of material,' says Jesper Jorgensen. 'Perhaps they were working for the same boss but do not operate together.'

Esko Hakli, the former director of Helsinki's University Library, also believes Perry must have had help and may have been working for an anonymous collector or a criminal organisation. He points out that when Perry needed to stand bail following his arrest he had no difficulty raising £10,000. Mysteriously, two of the six maps he had stolen were then posted anonymously to the Finnish embassy from Cardiff. When Perry arrived in Finland to stand trial in August 2001, he arranged for the return of two further maps - a Ptolemy map of Britain and a Jansson map of North America. However, he never recovered the 1486 Ptolemy Mappa Mundi.

Perry's trial provided a rare glimpse into the mind of a map ripper. His technique was deceptively simple: he had razored the maps from their bindings using a small knife meant for sharpening pencils.

Then, after leaving the library he'd mailed the maps to himself in England. In his defence, he criticised the library's poor security, saying it was this that had 'tempted him' to steal.

The court did not buy his excuses, sentencing Perry to 18 months and ordering him to pay the library 52,000 euros (£36,000) in compensation. But incredibly - because his sentence was less than two years - he was not immediately taken into custody, and fled Finland soon after.

As the months have gone by without news of Perry and Bellwood's where-abouts, the Danes have grown steadily more frustrated. 'Why can't the British police find a British citizen? Frankly, we had hoped for more progress by now,' says Nielsen bitterly.

The only good thing to come out of the affair as far as Nielsen is concerned is an agreement known as the Copenhagen Principles, under which European libraries and police forces will undertake to exchange information about the activities of rippers more promptly. As Nielsen puts it, staring out the immense windows of the Black Diamond, 'Institutional silence can only benefit the thief.'