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.