One breezy afternoon in 2001, two friends of mine, Richard and Dido, were mooching around a building site in Cambridge when they came across a battered yellow skip. Inside were 148 handwritten notebooks. Some were crammed into an old bottle box that had jaunty green print on the side: “Ribena! 5d!” Most were scattered across the bricks exultantly. A few had royal emblems from George VI’s time. Others were bright, bubblegum colours, tangerine and mushy-pea green. A chalky jotter that Dido picked up broke like chocolate. Inside, the rotted pages were filled with urgent handwriting. Running up one of the margins were the words, “Hope my diaries aren’t blown up before people can read them – they have immortal value.” There was no name or return address on the books. The diarist was simply “I” who had lived, and then died, and been pitched in a skip.

What could my friends do? They couldn’t take them to the police: they would laugh. They couldn’t leave them in the skip: that would be criminal. I’m a biographer, so Dido dumped them on my doorstep. Why not, she said, write about an anonymous diarist found in a skip? It would be the first ever biography in which the biographer hasn’t a clue who his subject is. Dido had left the books in three boxes, one of which had a label on the side addressed to the librarian, Trinity College, Cambridge.

Perhaps the diaries had belonged to a Trinity don, I thought, and got depressed. I slid the boxes down the corridor to my study and shoved them under a table. I think it was because they looked so interesting that I didn’t want to read them. I was deep at work on a biography and didn’t have time to get interested in anything new.

When I left Cambridge and moved to London to housesit for a pianist, one of the boxes became a cocktail table, another propped up a chair and the third, too wonky to be of any use, got kicked under the Steinway.

Two terrible things happened during those five years in London. Dido, my writing collaborator for 25 years, was diagnosed with pancreatic neuroendocrine cancer, the same disease that killed Steve Jobs. It had seeded over her liver. Its spores were crowded in her blood. Richard, a well‑known professor of the history of ecology – a field he had almost invented – had a car crash and was grotesquely brain damaged. He was now in a wheelchair, unable to speak.

The bloodbath diary was from 1961: greeny-blue and caved in halfway up the spine

Occasionally, I’d peer inside one of the boxes. But I always felt slightly appalled. The books marked a time when Dido was well. They emphasised that she might be dying. They were hateful.

In 2011, my girlfriend and I moved again, to Great Snoring, Norfolk. I’d almost forgotten about the boxes by then. They were just three more of the hundreds we drag about every time we change landlords. I shoved them in the back of the van with the rest, yanked them out among the chickens and runner ducks at the other end, and dropped them into a storeroom. At which point the wonky Ribena box burst open and spilled out its diaries. I couldn’t help but read some of the pages in front of me. One of the first I picked up featured a stabbing, followed by a bloodbath.

***

One of the oddities of reading the diaries of someone you have never met and know nothing about, not even their name or sex, is that everything seems clear to you before the end of the sentence. Even the measured way the writer records the date in blunt, soft pencil at the top of the first page, in square brackets, is enough (together with that label on the side of the box) to bring up a distinct image of him in his book-lined Cambridge college rooms, bent over his desk, brow furrowed, cataloguing the volume with the same careful respect he gives to his collection of scientifically significant seashells.

The bloodbath diary was from 1961: greeny-blue, not much bigger than a jacket potato and caved in halfway up the spine. An inside page printed with useful information from the publisher calls New Year’s Day “the feast of the Circumcision”.

The diarist’s handwriting races into this volume on the first blank page and covers 252 sides and just four weeks of daily life. In the middle, “I” describes a stabbing: “Then, to my horror – a sudden burst of blood rushed from my body. Ran about, and outside the house calling for Nizzy desperately.”

Who has stabbed him? Why? Who is Nizzy? “I” doesn’t say. What time of day is it? It might be first thing in the morning, because “I” reports that he’s in his pyjamas. But then, on the same pages, “I” talks about being “an artist”, so it could be any time of day. He appears to be not in Cambridge but (for some reason) in Luton, and calculates that he will need a blood transfusion and returns to the house to ring an ambulance, “weeping with frustration” when he can’t get through.

Abruptly the squall ends. The bleeding stops. Nizzy comes home and turns out to be his mother. She tells him to stop fussing. Our mystery diarist hasn’t been stabbed, slashed his wrists or fallen out of a window into a greenhouse. He’s suffering “because of my sex”. The poor man is having his period.

He’s a woman.

Alexander Masters: ‘After just one hour of reading them, I was desperate to look at them again.’ Photograph: Pal Hansen/The Guardian

What man hasn’t wanted to gawp around a woman’s thoughts? It wasn’t just gloom and convenience that led me back to these diaries. It was eroticism. After just one hour of reading them, I was desperate to look at them again. I decided I wanted to find out who the diarist had been and why she had died and been thrown away.

I knew I should take all three boxes to Cambridge police station and, if they remained unclaimed, after a suitable time have them incinerated. I was a Peeping Tom to do anything else. The writer describes things in a way that makes it clear she never expected or wanted anyone else to hear about them, let alone put them in a biography.

Thrilled, I lit a fire, backed myself on to an armchair and kept reading. I could hardly believe my luck.

Where would “I” take me first? To my shock, it was the toilet.

“I”’s curse began when she was 14, took over her life when she was 20, at its worst ruined three weeks out of every four (one lost to fear, one to pain, one to exhaustion), caused her to lose around 36 litres of blood and membrane, and was not considered bad enough to need medical attention.

But whenever I fantasised that she was somebody famous, I felt immediately, and as decisively as if the books had been dropped on my head, bored. The great excitement of an anonymous diary is that it might belong to anybody. Even giving “I” a name destroyed a vital thing that made the books interesting – a sense of quiet universality. Give the diarist a name, and she became just another stranger who didn’t want to accept my gaze. Imagine that she turned out to be some celebrity and the books (and my voyeurism) became almost nauseating.

It says a great deal for the diarist that she managed to keep me reading. She remained, throughout the guided tour she gave me of her mind, honest, funny, outlandish and respectable.

She writes long letters to 'E' and gets terse, pompous replies: 'E said I am a weakling'

Nothing is certain. That’s the number one cancer cliche. Less than a year after Dido’s first course of chemotherapy, the tumours were back. It was difficult to tell which was murdering Dido quicker: nature or medicine.

To avoid thinking about dying, she and I increased the amount of work we did on each other’s manuscripts – both of us were writing types of detective story: she about the hunt for the bones of Saint Thomas More; me the hunt for “I”. I was now working on the diaries every spare minute of my time.

I discovered “I”’s first name from her lover, whom she calls “E” in the diaries. He first crops up when “I” is 19 and has a temporary job at Cambridge public library, but their first meeting occurred five years earlier. He was her private piano teacher: kind, supportive, good enough (he said) to be a concert pianist, and grotesquely irresponsible. He allowed a young girl’s adulation to get out of hand. He is also spiteful, petty-minded and a prig. “I” reports hundreds of his sayings: “E said I am a silly ass”; “E said I am stupid”; “E said I am 14 years old [this written when she was 20]. I am not ripe enough yet”; “E said I was weak in every way.” Over the 25 years of their intense, abusive relationship, he demolished her confidence and ruined her ambitions.

In the midst of his relentless attacks, he also gave away her name. “E said I look dreadful”; “E asked (moving a little from me), ‘Are you insane?’” “E said the epitaph on my grave stone will be: ‘Here lies Laura, who did nothing, went nowhere, was loved by nobody.’”

Laura.

I missed my nameless pronoun. An abstract that had a few minutes before floated everywhere had been crushed into a particular. I liked this woman, whatever her name. I enjoyed her clumsiness and her obsessions and her occasional desires for an outburst of violence. I thought I recognised a lot of her qualities in myself. I wanted to understand her. Biographers often report that they enjoy a private relationship with their subject that is (even when this is impossible, because the subject is dead) shared on both sides. So what if Laura was called Laura? Laura was everywhere.

At one point in the early 1960s, in her 20s, she was living in poverty in London. Like every young, healthy, intelligent, imaginative, gifted person, she was full of wild and impossible plans. The handwriting in these volumes is urgent. Some entries are thousands of words long. She is trying to capture every second of her day. Occasionally, pressed on by her excitement, her handwriting wobbles and she resorts to underscoring: “injure, atmosphere, doesn’t believe me!! so hungry! I’ll kill them!”

“One must live dangerously, take risks, or one otherwise is in an ordinary metier all along… I now see I can do it. IT MUST BE DONE!!”

She is working on something that “fills and dominates my soul”. But, as with all the things that matter to Laura profoundly, she doesn’t say what this Great Project is, either because it would be dangerous for her to do so, because she is a spy or a bombmaker; or because “it” is so obvious to her, so much a part of her, that “it” must be on a par with her existence.

She writes long letters to “E”, and gets terse, pompous replies: “E said I am a weakling. E said there is no place for them in life, they ought to be hung up”; “E said she’s glad she’s not my parents.”

She’s glad? “E” is a woman? It turns out that E stands for Elsa.

‘Unless I arranged the diaries, I couldn’t know how everything tied together.’ Photograph: Pal Hansen/The Guardian

That’s nothing compared with the next surprise. Elsa is 50 years older than Laura. I had to leap up from my bed and dab the walls to sop up my splattered tea after I read it. When they first met and Laura fell in love, Laura was 14 and Elsa 64. There was nothing lurid about Laura and Elsa’s love. Intense and erotic, it was never consummated beyond a chaste kiss, yet it was enough to command Laura’s life. When “E” died in 1979, aged 90, Laura was 40. She lost her closest friend, her mentor, her decision-maker, her personification of artistry and, for the next 20 years, herself.

Laura’s handwriting collapses with her spirit. She gives up her hobbies: music, films, bike rides in the country. The early diaries from the 1960s are written in ebullient letters. Five words are sometimes all it takes to fill the width of a page. After the death of “E”, Laura crams 14 words to a line. The height of her letters becomes the same as the thickness of her pen nib. She describes herself as “ruined”, “lost”, “sacrificed”. It is impossible to read more than a volume at a time of this miniscule script. I deliberately kept the diaries as they’d been recovered, early and late books jumbled up in no particular order. It kept Laura’s life fresh and lively; it was easier on my eyes.

After 1990, everything succumbs to television. She disappears as a human being in these last years of her life, and reappears as cataloguer of Michael Barrymore gossip. She rages against “those who are stuffed with sleep”.

Laura frequently refers to a man called Peter. He is her “gaoler”, a “cruel” person. It appears that Laura is illegally locked up in Peter’s house, “It was in the news that a man has been let out of prison – was wrongfully imprisoned since 1975, 23 years; myself been shut up at Peter’s for one year more.”

Laura’s confinement isn’t absolute. She can leave her room and the house; but she is back on her mattress by the end of the day. She is allowed to attend her father’s funeral, visit her mother, shop for food and clothes, spend an afternoon in the cinema. “Grinding” back and forth on her bike she visits the local Co-op and makes purchases: “A 50p bunch of watercress that had started to rot”; a liver casserole ready meal, which she boiled up “to make it safe”; the “remains” of a swede; a fat-reduced garlic dip for 15p (“if it isn’t nice, it isn’t a disaster”). On one occasion, discussing a Rosamunde Pilcher bodice-ripper that she’s bought from the market for a triumphantly small sum, we discover that Laura has even attempted erotic thoughts about Peter. The attempt fails. Is she suffering from a mild version of Stockholm syndrome?

He is, according to the diaries, in his early 70s and worth between £5m and £12m. Laura is not his prisoner. She is not the Trinity don gone wrong. She is his live-in housekeeper.

'Are you Alex?' she said. 'I was just writing in my diary about you'

And so the surprises leaped up from the pages of these gentle, quiet diaries. I employed two graphologists, a detective, an expert pianist (to explain “I”’s infatuation with “E”); some comically bad mathematics (of my own devising) to estimate “I”’s height from the curvature in the lines of her writing; a doodlefit portrait (like a photofit, but with drawings) based on her rare descriptions of her looks. “I have glorious, tremendously thick hair, shining in rich goldy red and red brown” (1961); “upslant eyebrows” (1963); “a round face like a full moon” (1993). Everything in, about and around the text was a clue. I wondered if careful scientific analysis could reveal whether the injuries the wonky Ribena box had sustained as it landed in the skip were because it had been hurled (perpetrator enraged) or lobbed gently (perpetrator calculating). I enjoyed my intimacy with this universal woman.

It maddened my girlfriend Flora to listen to me puzzle about Laura yet still not take the basic step of tipping the books out of their boxes, sticking labels to the spines with the date written on and arranging them in the right sequence.

But I was absorbed by my sense of possession. I insisted that the disorder in which I kept the diaries in their boxes was another clue that captured something about Laura’s state of mind that the 5m words written across 15,000 pages missed – although I was never quite sure what that something was.

Flora would listen patiently, wait a few more months, then make her point again: had I read all the diaries? No. Had I read above a third of them? No. So, I hadn’t studied them properly. Unless I arranged the books chronologically, I couldn’t know how everything tied together, and therefore could not make a proper study of the contents. So many of my unconscious assumptions about Laura had been false. How many others were?

I knew Flora was right, so for years I ignored her. Then one day I did as she said. It took me until 20 past midnight. Many of the 1990s books were not dated. The only thing to do was read through the tiny handwriting until you hit a TV celebrity death or a fresh court appearance by Michael Barrymore, and crosscheck on the web. Exactly as Flora suspected, I discovered two new facts about Laura.

The first is that my 148 diaries represent only about one eighth of the total number of volumes Laura wrote. It turns out that I don’t have a single complete year after 1962, and that almost all the 70s, the second half of both the 60s and 80s, and most of the 90s are missing. Estimating from the gaps in my collection, the correct total number of books is closer to 1,000, or 40m words. Laura was the most prolific diarist in known history.

The second fact: Laura is still alive.

Pages from one of the diaries. Photograph: Pal Hansen/The Guardian

A month later, I saw Laura Francis for the first time. She was standing in the doorway of her bungalow, clutching a ring-bound jotting pad in her right hand. “Are you Alex?” she said as I held out my hand to greet her. “I was just writing in my diary about you.”

She was exactly as I had come to picture her: tall, slightly stooped, her face “round as the moon”, and with lots of hair. She wore spectacles and looked bemused not just by me, but by everything beyond her front step.

I was delighted. I wanted to burst into tears.

She opened her jotter.

“Here it is: ‘As I expected, that Masters man has not given up. He sent a card saying he’d show up at the house on Thursday and Friday at 5.30, but unfortunately he hasn’t put a date on the card. It could be a fortnight ago or it could be today.”’

I’d discovered Laura was alive because, by putting the diaries in chronological order, I’d come across a book I hadn’t checked before. It was the last in the collection, dated August 2001, just weeks before Dido had clambered into the skip. Laura wrote that she was sitting in the kitchen after coming home from Peter’s cremation. She was about to be forcibly removed from his house. Ten minutes later, after spending £9.50 looking her up on the electoral register, I was staring through Laura’s bungalow window on Google Earth.

In person, it took me 20 minutes to work up the nerve to explain that I had read her diaries, knew all her secrets and, with her permission, wanted to publish a biography about her that would reveal everything. It didn’t perturb her in the slightest. Her reply was the most surprising of all the surprises Laura has given me since I first met her on the page 15 years ago. Without hesitation, she said, “I’d cooperate in anything you wanted to write.”

“It doesn’t bother you that everyone will then be able to read what was in your diaries, too?”

“There’s no point in writing it all down if nobody ever reads it.”

Laura, the author of the diaries, sees her biography being printed. Photograph: Alexander Masters

Now in her 70s, retired and living alone, Laura still spends an hour and a half a day writing her journal. She began when she was 12, because her parents gave her a pot of green ink for Christmas, and she likes green ink. Since then, “these little books full of heartbreak” have served many purposes: a refuge, a place to let her worst thoughts out for a runaround, “a form of prayer”. She continues writing them today, “because I enjoy the sound of the pen on the page”.

How did they end up in the skip?

She has no idea. She hadn’t even realised that they had gone missing.

Laura and I are now friends. She has read the biography I have written of her life twice, and approved it all.

“Is there anything in the diaries that you feel you wouldn’t want discussed?” I asked.

“Oh no,” she replied. “It wouldn’t be true otherwise.”

• A Life Discarded, by Alexander Masters, is published next week by Fourth Estate at £12.99. Order a copy for £10.39 from the Guardian Bookshop.