Jeanette Winterson once asked her adoptive mother  stringently immortalised in her first novel Oranges Are Not the Only Fruit  why they couldn't have books in the house. "The trouble with a book is that you never know what's in it until it's too late," answered the peerless Mrs Winterson. As advertisements for reading go, it's pretty seductive. But it also happens to be wonderfully true of this vivid, unpredictable and sometimes mind-rattling memoir. You start it expecting one thing  a wry retake of her working-class gothic upbringing  and come out having been subjected to one of the more harrowing and candid investigations of mid-life breakdown I've ever read. This book is definitely of the sort that Mrs Winterson feared most: truths that most of us find hard to face, explored in a way that disturb, challenge, upset and inspire. And so yes, by the time I realised what it was really about and what it was going to do to me, it was definitely far "too late".

Jeanette Winterson was adopted as a baby by a couple who had been hoping for a boy. An evangelical Pentecostal Christian who possibly had never had sex with her husband ("They're his only pleasure," she said of the three packs of Polo mints she gave him every week), Mrs Winterson weighed 20 stone and kept a revolver in the kitchen drawer. If someone knocked at the door she "shoved a poker through the letter box". As Winterson very memorably puts it, she "did not have a soothing personality".

When her adoptive daughter upset her  inevitably and often  she'd say that the devil had led them to the "wrong crib". The young Jeanette was frequently deprived of food or locked out of the house or into the coal hole. Secretly, she began to read. But when her mother discovered the stash of books, she burned them. Lonely and bereft, she fell in love. But when her mother discovered her in bed with a girl, she took her to church and subjected her to three days of praying and beatings  not to mention a chilling attempted sexual assault from one of the elders  to "exorcise" the evil spirits.

Finally, after starting another relationship with a girl  and this time facing a stony ultimatum from Mrs Winterson  Jeanette ran away from home. She slept in a friend's car, and then in a sympathetic teacher's spare room. The teacher encouraged her to apply to Oxford. The rest is history.

Many years later  and Winterson admits to skipping over the large middle chunk of her life  a long-term relationship with the theatre director Deborah Warner comes to an end and Winterson goes "mad". Realising that it is, in part at least, her own inability to balance and temper her craving for reassurance that has caused the split, she has what amounts to a serious breakdown and attempts suicide. A little later, and by now in love with the therapist Susie Orbach  who seems in some way to be the angel of stability and calm that Winterson deserves  she feels safe enough to start to follow the trail that has always both tempted and frightened her: the one that might lead to her birth mother. Pages, months and many legal and emotional tussles later, it does.

And here's where this book  which up to this point had been funny enough to make me laugh out loud more times than is advisable on the No 12 bus  turns into something raw and unnerving. It turns into something you need to read in private, simply because you can't tell what will happen next or what you'll feel about it (Mrs W, you were right all along). The idea at its core, that it's possible to get roughly halfway through your life and find that the things you thought you'd dealt with, laughed off, survived, have come back to wallop you hard  endanger you, even  feels urgent and universal. And Winterson, with her fierce impulse for honesty, seems determined to unpack it all at some personal cost.

All of this, you realise, is still very recent  it's current affairs, not history  and it involves real people. Some have their identities disguised and some don't. But if Winterson is open about others, she's also (typically) unsparing of herself, heartbreakingly so in fact. Her analysis of events feels swift and direct  barely processed at times. It does not make for an easy journey for the reader. It is very hard, for instance, to watch someone who clearly loves being alive as much as she does, deciding almost coldly to let herself die. And her response to meeting her real family  comically tentative, wary, needy, yet in crucial ways underwhelmed  is also bravely told. You catch yourself, breath held, desperately wanting there to be something good in it for her.

This book is a gamble, but then that's perhaps one of the less surprising things about it: Winterson has always been a risk-taking writer, instinctively tempering her own slightly bolshie directness with humour, compassion and kindness. All the same, I found myself feeling oddly protective of all these good people, the author included  hoping that their presence on these pages won't make them too vulnerable, hoping there won't be repercussions for anyone (hoping, in short, that the Daily Mail will stay away).

Of course, one of the book's queasiest ironies  and one you sense Winterson is fully aware of  is that it was Mrs Winterson who made her into a writer. By attempting to stunt her daughter's emotional and imaginative growth with fear and religion, she succeeded in doing the exact opposite. She created someone who learned to live in her head, and to love, trust and remember words: "Fuck it, I can write my own," was young Jeanette's thought as she watched her beloved books burn.

If this were a novel, you might leave it like that. But real life is a baggy old thing, never so straightforward, and one of this memoir's bizarrest moments  and most glorious contradictions  is the one where Mrs Winterson reads Jane Eyre aloud to a seven-year-old Jeanette, cunningly changing the ending as she reads, to have the hapless governess marry the sanctimonious St John Rivers. Megalomaniac passion-killer she might have been, but here was a woman who was clearly excited by narrative, who cared how things turned out, who was  surely?  fascinated by those unpredictable and dangerous things called books.

The triumph of this memoir is that, with understanding, intelligence and a verbal agility that leaves you in awe, Winterson dares, in the strangest way, to celebrate this. In fact its many sparkling contradictions are what make me love it most. As Winterson says when she realises that she doesn't like hearing her birth mother criticise Mrs Winterson: "She was a monster, but she was my monster."

Jeanette Winterson's memoir is written sparsely and hurriedly; it is sometimes so terse it's almost in note form. The impression this gives is not of sloppiness, but a desperate urgency to make the reader understand. This is certainly the most moving book of Winterson's I have ever read, and it also feels like the most turbulent and the least controlled. In the end, the emotional force of the second half makes me suspect that the apparent artlessness of the first half is a ruse; that, in a Lilliputian fashion, what appears to be a straight narrative of her early life is actually tying the reader down with a thousand imperceptible guy ropes, so that when she unleashes a terrible sorrow, there is no escaping it and no looking away.

"Why be happy when you could be normal?" is the real-life question of her adopted mother, as Winterson is evicted, at 16, for taking up with a second girlfriend (the attempts to exorcise her sexuality after the first having been unsuccessful). There are passages and phrases that will be recognisable to anyone who's read Oranges Are Not the Only Fruit: this is not surprising, since that first, bold announcement of Winterson's talent was a roman à clef, and never claimed to be otherwise.

So anecdotes and jokes crop up in both books: the mother says the lesbian sweet-shop owners deal in "unnatural passions", and the young Jeanette thinks it means they put chemicals in their sweets; the gospel tent, the CB radio, all the memorable details of the first fictional outing come up again, but the point is not that this is repetitive. Rather, that the documents are intended as companions, to lay this one over the last like tracing paper, so that even if the author poetically denies the possibility of an absolute truth, there emerges nevertheless the shape of the things that actually happened. I had forgotten how upbeat Oranges was; it may have been peopled by eccentrics, with a heroine held in alienation by the aspic of impotent childhood, but there were upsides. "I suppose the saddest thing for me," Winterson writes now, "thinking about the cover version that is Oranges, is that I wrote a story I could live with. The other one was too painful. I could not survive it."

The upbringing as she tells it now is far bleaker; she was beaten, she was often hungry, she was left all night on the doorstep by a mother whose religious excesses might even have been a secondary influence on the household the first being her depression, which was pervasive and relentless. She was not well loved. However, the story's leavened throughout by other observations. The geopolitics I sometimes found bold, and other times found too broad to be conclusive: "In a system that generates masses, individualism is the only way out. But then what happens to community  to society?" But it wriggles with humour, even as Jeanette describes Mrs Winterson, who, in between her violent homilies and dishonest violence, had like any good tyrant various crucial absurdities  "she was one of the first women to have a heated corset. Unfortunately, when it overheated it beeped to warn the user. As the corset was by definition underneath her petticoat dress, apron and coat, there was little she could do to cool down except take off her coat and stand in the yard." There is Winterson's quirky favourite hymn ("Cheer up ye saints of God," it starts, "There is nothing to worry about"), her loving, impressionistic descriptions of classic authors, from TS Eliot to Gertrude Stein, as she first encounters them. And even with all this new, distressing detail, the story of her childhood ends well  it ends in escape.

Then there's an odd page or two entitled "Intermission", which finishes: "The womb to tomb of an interesting life  but I can't write my own; never could. Not Oranges. Not now. I would rather go on reading myself as a fiction than as a fact  I am going to miss out 25 years  Maybe later "

And suddenly we are on to territory which is alarming, moving, at times genuinely terrifying; skip forward a quarter century, and Winterson has just split up from her girlfriend, the theatre director Deborah Warner. She finds her adoption papers in the effects of her dad, when he's moving to an old people's home. She has a nervous breakdown and attempts suicide. "My friends never failed me and when I could talk I did talk to them. But often I could not talk. Language left me. I was in the place before I had any language. The abandoned place." At times she describes the process with precision. Other times, though, the scars of this first abandonment are given in the most unadorned, uncharacteristic prose, as though she's trying to gnaw her way through her own sophistication to get to the truth of it. In a way, the presence in the narrative of Susie Orbach, with whom Winterson started a relationship just before she started looking for her birth mother, acts as a reassurance to the reader as much as to the author, a fixed point to whom we can return, whose very inclusion means that, whatever happens, a fresh abandonment won't be the outcome. Otherwise I genuinely think it would be unbearable. At one point I was crying so much I had tears in my ears.

There is much here that's impressive, but what I find most unusual about it is the way it deepens one's sympathy, for everyone involved, so that the characters who are demons at the start  her adoptive mother but also, to a degree, her acquiescent adoptive father  emerge, by the end, as simply, catastrophically damaged. In the process of uncovering that, she painstakingly unpicks the damage they wreaked on her. The peace she makes with her adoptive family is, in this sense, more important and evocative than the more complicated and double-edged peace that comes with tracking down her birth mother.