When a marriage fails, the protagonists usually look for an explanation that will shelter them in their respective differences. Paradoxically, this explanation tends to be developed in antagonistic proximity to each other: it ricochets against the familiar surfaces of the marriage in cruel mimicry of the dependency that is now being disavowed. This is normally what happens. Rachel Cusk, who has particular difficulties in amortising any bit of herself to what might be considered normal, is having none of it.

"My husband believed that I had treated him monstrously," she writes. "This belief of his couldn't be shaken: his whole world depended on it. It was his story, and lately I have come to hate stories. If someone were to ask me what disaster this was that had befallen my life, I might ask if they wanted the story or the truth."

There's something so vertiginously condescending in this statement that one is almost sucked off the cliff face of the page. Unable to rattle her husband out of his version, she scorns him as a demi-wit for needing a story in the first place. She has "come to hate stories" (how dowager this sounds), yet the context in which she imparts this information is page 2 of a story, this book called Aftermath, with its literary artifice and patterning and stories within stories  a trip to the dentist, baking a cake, a child dressed up as faun, a lodger howling in the garden at night. Indeed, Cusk's story is so important to her that she has created out of it a whole landscape, Cuskland, whose contours and features she has been mapping since her first memoir, A Life's Work (2001), about how pregnancy and motherhood stole her identity.

Cusk's declared interest in the truth does not encompass the low detail of how her marriage actually came apart. "An important vow of obedience was broken," she tells us, in a brisk aside that implies adultery but not by whom. When Cusk is told by her (female) solicitor that she has "no rights of any kind" and will be obliged to support her husband financially (he having left his job to look after the home and the children), she protests: "But he's a qualified lawyer. And I'm just a writer." To which the solicitor replies: "Well, then he knew exactly what he was doing." Passing as it does with no qualifying comment from Cusk, the sheer nastiness of this exchange leaves a stain.

There are obvious legal reasons for Cusk's incomplete treatment of such issues, breach of privacy being one with which she is already familiar (a threatened lawsuit against her 2009 memoir-lite, The Last Supper: A Summer in Italy, resulted in stocks having to be pulped a month after publication). But the facts, in any case, are not the same as the truth, may even be tangential to it. The truth, for Cusk, lies elsewhere, not in story but in history, in the notion of aftermath that she helpfully identifies (lest we fail to) as "the book's elemental theme".

Cusk's history teacher at school, Mrs Lewis, was a medievalist who celebrated the fall of the Roman empire as if it were a personal triumph. She relished the "darkness, the aftermath" of collapsed civilisations, when megalomaniacal administration and "conquering unity" had given way to the "disorganised life", where issues of justice and belief had to be resolved at the personal level (how refreshing, now you didn't need a permit for a crucifixion). In this vestigial world, Mrs Lewis revealed, mystics and visionaries, and scholar-monks sequestered in tenebrous libraries, were left to explore the mysteries of existence and nudge the world towards a new consciousness.

We are not privy to Mrs Lewis's explanation of what the rest of humankind was up to  that sizeable portion of it for whom the rigours of mere survival could not be exchanged for the privileges of the inkhorn  and one suspects that the teenage Cusk was not minded to linger on their fate. For she had already embraced the nostrum that ruination was a great facilitator, a catalyst for "the dark stirrings of creativity", in contrast to "civilised unity" that was "racked by the impulse to destroy".'

There's something unavoidably Pol Pot about this thesis, and Cusk's fascination with it comes at a very great price. There is no more dangerous illusion than the fancies by which people try to avoid illusion, and nothing more shocking than discovering that this pertains to you. "Why had I destroyed my home?" she asks at one point. It's a terrible and brave question, loaded as it is with the comprehension of having fatally misunderstood something, but what? And then: "I cannot remember what drove me to destroy the life I had. All I know is that it is lost, gone." There she stands, the "untenanted wastes" extending before her. Year Zero.

It's not a congenial place, this Cuskland, with its low mephitic cloud of complex melancholia. If we were only there to witness Cusk dispose of body parts we would scurry away. What detains us is her cool, clinical examination of the remains, the truths that are returned when she scrapes at the marrow of experience. "There is at first a consumptive glamour to suffering," she discovers. "I don't eat, for fear that nourishment will hurt me with its inferences of pleasure." She meets with her solicitor, who is petite, neatly tailored: "I was thin and gaunt with distress, yet in her presence I felt enormous, rough-hewn, a maternal rock encrusted with ancient ugly emotion."

Lying in the dentist's chair she registers the "pulse, the very heartbeat and hydraulics of the day" in the streets below; driving past hedgerows she notes "their mysterious convoluted interiors"; the ring of the telephone discharges into "the smashed days of late summer". This is as good as Updike, and follows his precept of rendering "the small authentic thing over the big inflated thing". Unfortunately, Cusk can't resist the latter. She should have thinned the clots of classical myth, and dumped altogether the bizarre final chapter, with its utterly disingenuous novelistic trick of resolution. This is writerly greed, swooping on everything and wringing meaning from it, transforming it into something else rather than just letting it be. "Life is other than what one writes," André Breton cautioned. Cusk recognises the pathology, struggles to escape it. "I don't want to tell my story," she groans. "I want to live."

She is not a whinger, she does not ask either for pity or approval. Responding to the charge that she had denigrated motherhood in A Life's Work, she wrote, in the introduction to a new edition, "the book is governed by the subject I, not You  I am not telling you how to live; nor am I bound to advertise your view of the world." This is the code of honour in Cuskland: do not to surrender to the opinion of others; the only legitimate identity is the one you claim for yourself; better to die on your feet than live on your knees.

This separatism  ma guerre à moi  has drawn some exceptionally vituperative criticism, much of it driven by the perception that Cusk is placing herself above the social contract rather than outside of it. As a feminist, she wishes to live by an ethos of parity, and to this end she strikes a treaty with her husband whereby "we would live together as two hybrids, each of us half male and half female. That was equality, was it not?" The concordat collapses when she realises that the "authority" of marriage itself is oppressive, that "the cult of motherhood", with its "sentimentality and narcissism", is somehow "anti-feminine" and makes her feel "unsexed". Yet when the matter of custody arises, she shocks herself by invoking that which she most disparages, "the primitivism of the mother, her innate superiority, that voodoo in the face of which the mechanism of equal rights breaks down". She says: "They're my children. They belong to me."

The contradiction, the double-standard for which she is so energetically despised is not denied by her. It is mercilessly exposed and anatomised. That's the point: behold the mess, the aftermath. She's a narcissist? Undoubtedly. But then so are we, in our fury that she does not apologise or offer a smooth surface to reflect back an image of ourselves and the contracts we have entered into. The subject is I, not You.

 Frances Stonor Saunders's The Woman Who Shot Mussolini is published by Faber.

Rachel Cusk's books are like pop-up volumes for grown-ups, the prose springing out of the page to bop you neatly between the eyes with its insights. The opening words here are plain enough  "Recently my husband and I separated"  but by the bottom of the page we are treated to: "An argument is only an emergency of self-definition, after all." On the next page, we are informed that "my husband believed that I had treated him monstrously" but even as the prying fingers of the mind come up to twitch the net curtains of context, Cusk's cool paw slaps them down again.

Graham Greene famously said that all writers need a chip of ice in their heart; Cusk can come across as the most beautiful ice palace of stalactites and stalagmites, and some people find her company, albeit by proxy, about as inviting as a long weekend in a walk-in frigidaire. We are used to female writers who use their private lives as unmitigated material being somewhat hormonal; this somehow "excuses" what might be seen as a highly unfeminine ability to turn their personal upsets into money. Cusk doesn't pretend for a moment that she couldn't help herself or doesn't know what came over her when she renders down her marriage into material; she does it with all the care and deliberation of a monk illuminating a medieval manuscript.

Though ostensibly both working the somewhat seedy seam of female confession, Cusk is the exact opposite of Liz Jones, which is surely one of the most lavish compliments I have ever paid anyone. This is no sob story of cat-coddling and sperm-stealing but rather a steely refusal to bare all while reporting forensically on the anatomy of a divorce; a sort of dance of the seven veils in reverse. In a Jones book about the end of a marriage, Offa's Dyke would be nothing more than a nasty name given to an adulterous fling of her ex-husband's; here, it's a cue for a quick tangential trip through the intricacies of Saxon power-broking. Raised on the show-and-yell salaciousness of our celebrity skin culture, a person may at this point crack, and exclaim crudely: "Bitch, please  spill!" But settle down and get used to the idea that you're going the scenic route, and a highly rewarding experience will be yours.

Some say that Cusk has no sense of humour, but expecting giggles from this writer would be akin to expecting sonnets from Benny Hill. I was a little disappointed, though, that she doesn't even attempt to capture the relief, joy and even wonder of divorce, at least on the part of the instigator (which she was). Much sympathy is given to the "innocent" party in a marriage  the one who is left  but no one bothers to put themselves in the shamed shoes of the poor bolter/adulterer, who must perform the thankless task of peeling the clinging body of the unwanted spouse away like a dead Siamese twin.

The (self) righteous zeal of the innocent party is a grotesque spectacle; like Cusk, I have had a dismissed husband accuse me of being a disgrace to feminism because I refused to give in to his every demand. Seeing a Wronged Man acting like Violet Elizabeth Bott with PMT is never a pretty sight, and even Cusk's monumental self-control can falter in the face of it: "I earned the money in our household, did my share of the cooking and cleaning, paid someone to look after the children while I worked, picked them up from school when they were older. And my husband helped me why, exactly, was it helpful for a man to look after his own children, or cook the food that he would eat?"

A writer I really love expressed surprise when I told her how much I admired Cusk, offering the opinion that "She can't say what she thinks, only retrospectively garlands it with all sorts of whimsy so she sounds super clever and fragile. I blame Joan Didion." And there is a chapter about toothache and a trip to the dentist which reads like a Craig Brown parody of the over-examined literary life. There is even a bit of banality going begging  "Everywhere people are in couples" sounds like someone is trying to keep down with the Joneses, both Liz and Bridget  which is surely a first for Cusk. By the time sunshine was streaming through the windows "like sun falling on a ruin", the boiler was "choking and grumbling cholerically" and the plaster is "bulging and flaking like afflicted skin", I was beginning to get whiffs of Cold Comfort Farm, which I'm sure wasn't the over-egged effect aimed for.

There's also a rather odd last chapter, seen through the eyes of a hapless eastern European au pair living with a disintegrating family, which had me somewhat confused. I was always perplexed by those ancient drolls who would presume to have an audience in stitches by the act of "throwing" their voices, and this left me similarly baffled. But, on the whole, this is a predictably brilliant book.