Candia McWilliam began going blind in the spring of 2006 (she was judging the Booker prize at the time, and remains alive to the bleak comedy of the timing). She was soon diagnosed with blepharospasm  a condition that made it first difficult and then impossible for her to open her eyes. The affliction would be terrible for anyone. For McWilliam, whose sense of pleasure, direction, value and reward in life was greatly to do with reading, it was an especially bitter blow. This memoir gives an account of how she coped, laying its narrative of suffering over the story of her life, and leading it eventually towards the present  and a fragile recovery of sight. It is in some parts extremely sad, in some indulgent, in some brilliantly written, in some comically jewelled, in some shrewd, and in some surprisingly naive. Some readers will find it indigestible; others will be persuaded that its less successful elements speak powerfully to its strengths in order to create an unusually revealing self-portrait.

McWilliam's prose has always courted  or been the occasion of  controversy. Although her previous three novels and one collection of short stories have described a gradual transition from elaboration to comparative simplicity, her style remains, as it has always been, high. In the early days this meant she was often accused of pretension: Pseuds Corner thought it was Christmas every time she opened her mouth. Here she has taken another stride towards plain speaking, though she still loves to build her sentences on tall arches; she still drops names; she still writes about money as though everyone had buckets of the stuff ("Olly's godfather . . . had just inherited somewhere off the King's Road"); and she still makes observations in terms that will make some readers want to join the Communist party ("I could smell that she had quinces about her person").

All these traits might be ridiculous in isolation. Yet, because they are the products of a clever mind and a generous sensibility, ridiculous is not all they are here. McWilliam knows perfectly well that she has a reputation for having "swallowed the dictionary", yet she continues to relish extravagant language: this may be stubborn, but it is also brave. She knows her blindness has shadowed a life that was in many ways privileged  yet she persists in making no apology for it: this may be provocative (or worse), but it is also a way of remaining true to herself.

In this book, McWilliam projects an image of herself that consists of opposites and paradoxes. The most conspicuous and puzzling have to do with the blend of experience and innocence. After McWilliam's mother died young, her father (a "classically educated modernist" who worked for the Scottish National Trust) married again; it was a comfortably discomforted childhood. At school (Sherborne) and university (Cambridge), she knows that she is beautiful but can't easily believe it because her weight is volatile and her height unusual. She understands that she is smart but doubts her capacity to make and keep friends. (At many points she writes of "my own sense of my unloveability".)

In those circumstances, McWilliam became a hungry reader, plundering the dictionary to defend herself and to impose herself on events while accommodating them within her descriptions. Her situation also prompted the search for a substitute family  ideally, one that had the advantage of being unlike her own in terms of size, affability and amusement, but also of being pleasingly remote and therefore captureable. The one she found fitted the bill precisely: they were called Henderson and lived on the remote Scottish "private" island of Colonsay.

The good Hendersons provided a way for McWilliam to control the world while withdrawing from it; they gave her authority as well as respite. Her more solitary expeditions were less successful. After university and a spell working for Vogue, she was briefly married to Quentin, Earl of Portsmouth, then for rather longer to Fram Dinshaw, an English don at Oxford. She emerged from these relationships with three children, a house in Oxford and an extremely serious drink problem.

In a more conventional memoir, written under less constrictive physical difficulties, the narrative of McWilliam's life would not be especially interesting. As it is, the bones of the story seem much less the occasion for her book than do the means of reviewing them (through the crevices in the blindness), and the opportunity to disclose psychological explanations. Here again, there is an interesting mixture of opposite forces. McWilliam is acute about the effects of her drinking, and of her blindness, and writes movingly about how they affected her life, but there is a curious reserve about underlying needs, compulsions and characteristics.

Sometimes this reserve appears simply as silence: we hear almost nothing, for instance, about a third important partner in her life, Mark Fisher. Why not? Because he asked her to keep quiet? Because she doesn't want to admit to him? We are not told  which seems odd, in a book so concerned to lay things bare. More often, the withholdings are of a differently complex kind. The most complex of all occur in her treatment of Dinshaw. We hear a good deal about his background (Zoroastrian, Pakistani), a great deal about his mother (domineering, interfering), and even more about his cleverness. We are also told, a wearying number of times, how she dotes on him, even when their marriage has ended and he is living with another woman (with whom McWilliam has a good relationship).

A memoir can send love-messages to an absent other and still keep the sympathy of its readers. What is unusual here is the gulf that opens between her insistence on her ex-husband's appeal and his characteristics as she presents them. This begins to emerge in the first part of the book, when Dinshaw allows his mother to drip poison into the "deep flaws" of their marriage. In the second and less focused half, as McWilliam takes the tortuous road back towards sobriety and sight, it becomes impossible to ignore.

This creates an interesting conundrum at the centre of the book: the character we are meant to value is presented to us in terms that make him hard to like. If What to Look for in Winter were a determined assassination job, this would be understandable. But it isn't. It's a book written out of sorrow and pain and love. A book that, for all the brilliance of its author, doesn't seem completely aware of everything it has revealed. The prose may be highly self-conscious, the syntax elaborate, and the references fancy. But the heart beneath is raw and bleeding.

Andrew Motion's The Cinder Path is published by Faber.

Candia McWilliam's long and lacerating memoir is an intimidating sort of a book. The names alone are enough to set the teeth on edge. Everyone in it seems to be called Ivo or Quentin, Leander or Annabel. Granted, a Rita passes through its pages and so, too, does a Graham, briefly. But since these two are cats, I don't think they count. You will find within its pages an earl, and a rather grand-sounding Oxford don; a private island, and a little girl who falls in love with Odysseus at the age of six. You will also find sentences that require re-reading several times, the better to unpack their meaning, the full extent of their admission, and their evasion. McWilliam's writing is a devilish mix of the plain and the filigree, the Scots in her always fighting with a wilder, more extravagant instinct; its sui generis beauty will not be to everyone's taste.

Nevertheless, it is worth setting aside one's inverted snobbery. What to Look for in Winter tells the story of the author's struggles with alcoholism, blindness and writer's block. It also tells you a great deal about families, their warmth and their chilliness, and how to survive them. Most of all, though, it is an extremely sagacious book about loss, and since we are all destined to be losers at some point, this would seem to make it  as they no doubt say over at Channel 4  a direct mail shot to anyone who has ever picked up a hardback. It is the book's wisdom, modest and hard-won, that will stay with you, not its Iris Murdoch characters with their witty put-downs, their big and possibly draughty houses, their Homer and their George Herbert.

In 2006, when she was judging the Man Booker prize for fiction, McWilliam started to go blind. It wasn't that her eyes had failed; it was her eyelids, which kept closing, that were the problem. Soon, she could see the world only in slivers, by holding her eyes open with finger and thumb. It sounds like something from Greek mythology, a psychosomatic condition, or a punishment, and McWilliam certainly seems to have felt, sometimes, that it was the latter.

In fact, its cause was a rare brain condition called blepharospasm. What to do about this curse? Towards the end of her book, McWilliam undergoes complicated and painful surgery that will, eventually, restore to her a measure of sight. But her first course of action is intuitive, even atavistic. She will try to write  or, at any rate, dictate  her way out of it. She has an instinct that her blindness is connected to what the shrinks like to call "suppression", but what the rest of us (I mean those of us not yet infected by Oprah Show incontinence) call embarrassment, or good manners, and consider a prudent saving of our emotional energy. Her memoir will, then, be an attempt to find her temper "in order that I may lose it, and in losing it, perhaps, find my lost eyes".

McWilliam's last book, a collection of short stories, was published in 1997. Now, suddenly, she is writing again. Of course, autobiography, as she repeatedly tells us, is not her thing. When she was still drinking heavily, it was an editor's somewhat unclassy suggestion that she write a misery memoir about alcoholism that sent her on her final, massive bender. But you can feel her developing a taste for it, all the same. For all that she is blind, for all that she resists full disclosure (my sense is that there are some things she simply cannot bear to write, and a few she was forbidden), there is true beadiness here. She despatches people  among them a female relative who gives her 40 rotten eggs by way of a 40th birthday present  with speed and ruthlessness. Quite a few people do not come out of this very well.

Her story is both magical and sad: it has fairytale qualities, by which I mean that it features an enchanted tower and a (semi-) wicked stepmother, but it is also distinctly modern, including, as it does, the most curiously and unfathomably "blended" family in all bohemia. McWilliam was born in Edinburgh, a city that was the wellspring of all her best qualities (her uncomplaining Protestantism, her whelk-eating thriftiness) and her worst (no one is harder on Candia than Candia). Her father was an architectural historian; her mother was beautiful and clever, but also thwarted, fragile and unhappy. At 36, she killed herself, leaving behind a tiny, bewildered daughter who liked reading, and who strongly disliked moving around. Quickly, too quickly, her father remarried, a Dutch woman who put Candia on an exercise programme in order to shift her fat. Wanting to escape, Candia elected to attend boarding school, and here the pattern began. Feeling herself to have been a cuckoo in the nest at home, she fluffed up her brown feathers and nestled in elsewhere. Once a cuckoo, always a cuckoo. At school, she met a girl whose family owns Colonsay  they shared a passion for junket  and soon began holidaying there, adopting her friend's parents as her own. When she married her first husband, the heir to an earldom, her Colonsay daddy gave her away while her real, Edinburgh daddy sat quietly in a front pew.

The marriage to the future earl did not last. Nor did her second, to an Oxford don called Fram Dinshaw. McWilliam roundly takes the blame for what a friend once described in his diary as her "bolting", but the reader feels this to be unfair (in the case of Dinshaw, her snooty Parsi mother-in-law could not have been less loving if she'd tried). McWilliam, whether she knows it or not, is always fighting a greater loneliness: the loss of her mother, the break with her father, her deracination (she misses, in her bone marrow, Scotland, and I don't blame her). The drinking began in her room at Cambridge, where she was brilliant and beautiful but still cuckoo-ish, with a bottle of sherry she was supposed to drink with other girls, but in fact sank alone. In her worst years, before rehab, she would glug anything, including Easy Iron ("not a smooth drink").

I don't want to make all this sound gruelling. When McWilliam is at her best, she can do something remarkable with words. I will never now be able to make coffee without thinking that the amount I should put into the cafetiere should weigh "about as much as a squirrel's tail". And there are funny bits. She befriends a tramp called Stanley. On the way to get the paper, she sees that he is reading a book. "Is it any good?" she asks. "Don't look, Candia," says Stanley. "I'm masturbating." Another time, she meets Christopher Hitchens on a New York street. It's Halloween. "What are you dressed as?" he asks. "A parsnip?" As a student, she lived on "rice cooked with Bovril or tomato ketchup or, when the occasion demanded protein, whelks"; these whelks I find hilarious, don't ask me why.

But still, it's painful, the gap between what you feel, and what she feels. McWilliam finds her temper, as she was hoping, but is never able to turn it on anyone but herself, which is unjust. I hope she knows that she is not some "maimed domestic animal"; that she does not remotely resemble a stuffed sea lion, one with "sawdust and kapok spilling at unseemly seams". The truth is that the behaviour of some of those around her would make anyone feel inadequate and lonely, and I suspect that she knows this, too, else she would not have shared it with us. That wisdom I mentioned earlier lies here, especially, in the things she does not say. She trusts her reader to fill in the gaps. "Do not underestimate the silences or breaks in a line," she urges. Well, I don't. Here, such quietnesses surely amount to silent screams.