Book-lovers around the world have been helping investigators trace thousands of rare volumes looted from one of Italy's oldest libraries by a gang of thieves including the librarian himself. While most have been recovered, a number of invaluable 15th and 16th Century books are still missing.

Inside a 16th Century church complex in the heart of Naples, the Biblioteca Girolamini's wooden shelves rise up and up towards richly decorated walls and vaulted ceilings.

They once held works of extraordinary value. There was a 1518 edition of Thomas More's brilliant and mysterious Utopia. Galileo's 1610 treatise Sidereus Nuncius, containing more than 70 drawings of the moon and the stars. And Johannes Kepler's study of the motions of Mars, Astronomia Nova, described as one of greatest books in the history of astronomy.

But this magnificent piece of Italy's cultural heritage was methodically plundered. Thousands of antique texts disappeared.

"Our investigations found that there was a true criminal system in action," says Major Antonio Coppola, a police chief who is leading the operation to recover the stolen texts. "A group of people... carried out a devastating, systematic looting of the library."

It was an art historian and academic, Professor Tomaso Montanari, who first alerted the police to what was happening. The library had been closed to the public for years, but Montanari heard reports that it was in trouble, and managed to make a visit with a student he was supervising in the spring of 2012.

Four books still missing Thomas More's Utopia (1518) - traced but not recovered

Kepler's Astronomia Nova (1609) - traced but not recovered

Galileo's Sidereus Nuncius (1610)

Dante's Divine Comedy printed before the 16th Century

He was shocked by what he found.

"There was a dog roaming around the library with a bone in its mouth!

"There were books spread around everywhere - on the floor, on the stairs, on tables. There was garbage - soda cans and papers - on the floor. It was total confusion, a situation of major decay. One of the library's members of staff took me aside, away from the CCTV cameras, and said: 'Professor, the director has been looting the library!'"

The director was Marino Massimo de Caro.

Maybe in a few years - when all the attention on the looting of the Girolamini calms down - these books will re-emerge Major Antonio Coppola

Montanari wrote an article for the Il Fatto Quotidiano newspaper, arguing that having De Caro as librarian was like having an arsonist in charge of a forest.

"The day the article was published, De Caro called me on my mobile and told me: 'You should be ashamed, you'll pay for this!'" Montanari says. "I was very scared. I called the police."

An investigation was launched, and De Caro was arrested.

"What strikes you was the complete lack of respect for these precious works," says Antonio Coppola. "Many of them were thrown on the floor. Chaos reigned... Shelves in which very important books once rested were now totally emptied."

He describes De Caro as the head of a ring of thieves, who would go to work in the evenings after turning off the library's limited CCTV monitoring system.

"They took the books off the shelves - dozens and dozens of them. Some were given particular attention - those which would have more market value... Then it was just piles and piles of books. They would put them in boxes, which were taken out to vans."

Seals identifying the manuscripts as part of the Girolamini collection were removed. In some cases they were just torn out, leaving the book seriously damaged.

"Sometimes they would even remove the binding of the book," says Coppola.

"They wanted to make it impossible to trace them back. But some of the books had binding from the 17th Century, which in some cases made the binding more valuable than the book."

Stripped of evidence of their origin, the volumes were put on the Italian and international markets.

Girolamini library The oldest library in Naples, founded 1586

Holds 150,000 volumes including 5,000 from the 16th Century and 120 books printed before 1501, known as incunabili (Italian for "cradles")

(Italian for "cradles") The library of choice for Giambattista Vico (1668-1744) author of New Science

Five-hundred books went to a German auction house, which gave the thieves a million euros (£840,000 or $1.4m) in advance for the batch. They would have received more cash if the sale had gone through.

"This was only one load of the books," says the police chief. "So you can imagine how much money they could have made from this looting if we and the courts hadn't stopped it."

Some could have been sold for hundreds of thousands of euros, some for tens of thousands, so a reasonable overall guesstimate, he suggests, may be in the tens of millions of euros.

In March this year De Caro was sentenced to seven years in jail, commuted to house arrest because he co-operated with investigators. Several other defendants were given lesser prison terms.

De Caro's lawyer says her client maintains that he did not take all the books that he is alleged to have stolen, and an appeal is planned.

In a statement to the BBC, his family insisted that he had worked for the good of the library - pouring his own money into the improvement of the catalogue and other projects - in the hope of being able to re-open it to the public.

But in court he confessed to stealing from the Girolamini.

And the police say that the gang in fact destroyed the catalogue, in another attempt to cover its tracks.

The big question is how someone like De Caro could have been put in charge of such an important institution.

Antonio Coppola says De Caro was a well-known figure in bookselling circles, and that he "had some kind of political cover that enabled him over the years to reach positions of responsibility in institutions more or less controlled by the Ministry for Culture".

Four books already recovered Giordano Bruno's Artificium Perorandi (1612)

A book by Copernicus (1566)

Leon Battista Alberti's De Principe (1520)

A copy of In hoc volumine heac, by Capitolinus and others, printed in Venice in 1519

He had no relevant academic qualifications.

The Ministry did not have complete supervision of affairs at the library, which is housed on church property, but it did approve De Caro's appointment in 2011.

And it concedes that "surveillance was not adequate" while the Girolamini "progressively decayed" - a process that began before De Caro's period in charge.

But remarkably, just 18 months after the looting was discovered, police investigators believe that they have already managed to track down the great majority of the stolen books.

"We think a high percentage - up to 80% - has been recovered," says Coppola.

"This is thanks to investigative activity - shadowing and wiretapping, for example - that has enabled us to identify movements and places were books were being kept. Warehouses, underground parking places, houses...

"We've had important collaboration with associations of antique booksellers. We've warned them to keep an eye open for books that they might have even the slightest suspicion could have come from the library."

Coppola has been contacted by concerned traders and collectors from across Europe, the US and Latin America.

"My personal email address is spread around the world among those in this sector, and I'm often reached by people that have doubts about a book," he says.

They are in an amazing state of abandonment and decay Fabrizio Govi, head of the Antiquarian Booksellers' Association, on Italy's libraries

Among the many books gathered up were some returned by the former Italian Senator and well-known bibliophile, Marcello Dell'Utri, a close associate of former Prime Minister Silvio Berlusconi.

"He received some books as a free gift. As soon as he realised from newspaper reports that they probably came from the Girolamini Library, he contacted the authorities and gave them back," Dell'Utri's lawyer, Giuseppe di Peri, told the BBC.

But one book, a 1518 edition of Thomas More's Utopia, he has yet to return.

"Regarding the other book, we are speaking about a book of very little value, and he simply can't find it at the moment. As soon as he does, he will give that back as well," says Di Peri.

Major Coppola, however, regards this book as particularly valuable.

He believes that more books from the Girolamini will surface in due course.

"In this job you learn that you need to adjust your attitude to time," says Coppola.

"In the illegal cultural assets market things move very slowly.

"Maybe in a few years - when all the attention on the looting of the Girolamini calms down - these books will re-emerge. And we like to think that at that point we carabinieri will be able to intervene."

Meanwhile, the Ministry of Culture says that it has plans to restore and re-open the library in a process that could take a year to complete.

But the head of Italy's Antiquarian Booksellers' Association, Fabrizio Govi, has his doubts.

When it comes to protecting the nation's cultural heritage, he says, its libraries have usually been at the bottom of the list of priorities.

"They are in an amazing state of abandonment and decay," he insists.

He fears that nothing will really change, and that the magnificent old Biblioteca Girolamini will never be properly restored to its former glory.

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