Much of Julie Myerson's writing, from Sleepwalking onwards, is interested in families broken by the sin or neglect of a mother with whom the reader is asked to sympathise. The geographical and historical settings vary  the Victorian London of Laura Blundy, modern Paris in The Story of You  but the narrator haunted by her failures as wife and mother appears in various guises.

In some ways, Then is different. It's set in the near future, or perhaps  if the teenagers' argot is a barometer  in an alternative present, a post-apocalyptic London where frozen bodies lie piled on icy streets in front of looted and burnt-out buildings in scenes reminiscent of wartime Leningrad in Helen Dunmore's The Siege. The exact nature of the catastrophe is unclear: one February morning, we learn, the temperature soared until garden ponds boiled so that "the [gold]fish bobbed, some of them split right open, the edges curling back as if on a hot grill". And then the ice age sets in. We meet the narrator walking the streets of the city, having forgotten everything about herself and her previous life, and follow her to the empty office block where she lives with people she may or may not remember and some other presences who may or may not be "real". The story of how she came to be there unfurls through fragmented flashbacks that leave a finely judged proportion of narrative work to the reader.

First-person narrators are almost by definition unreliable, and that's part of the pleasure for readers. But an amnesiac, nameless first-person narrator removed from her social and domestic context is a challenge, even before we begin to witness her apparently psychotic behaviour. For the first part of the book, the narrator repeatedly injures a young boy on a whim and feels no regret when others remind her of what she has done. She returns to the office block to find the child lying at the bottom of the stairs: "And there's a nasty ripening bruise on the kid's head. And maybe a cut as well." Her companions are waiting for her to apologise for throwing the child down the stairs: "Am I sorry? Am I? I try to think about what sorry might feel like and whether or not it's in my mind right now. Sorry. No, I do not think that sorry will come." This is not  until much later - a character with whom it might be possible to identify.

The writing is sparse, much of it dialogue. No metaphors, few similes, nothing to sketch an aesthetic underpinning for the narrator's bare reportage, no reaching for common ground. The curt, factual sentences give an effective sense of a damaged and dysfunctional mind, but they repel emotional engagement with the novel and offer no context for an intolerable situation. Even at the end, when we understand more about the narrator's trauma, her voice remains cold; it's the plot, not the character, that calls for empathy.

Nothing much happens in the novel's present tense, certainly nothing that hasn't already been done by Dunmore or by Cormac McCarthy in The Road. There are some nice touches  chocolate money prized more highly than banknotes, the narrator's inability to distinguish one ruined London high street from another because the landscape "is both familiar and not familiar. A sign says All Bar One and another says HSBC and another says Lloyds"  but the makeshift means of survival in a post-apocalyptic city are not novel (and I couldn't help wondering about the water supply in a tower block with no power or heating). The Siege is partly a domestic tragedy, but it's also a book about war and power and the relationship between the individual and the state. The Road is partly about fatherhood and love, but it's also about the logical conclusion of modern capitalism. Then doesn't, I think, have such ideological interests. The real story is in the past, before "there was a sound like a blade tearing through the sky", and we don't begin to hear this story until almost halfway through the book. The final disclosure is certainly horrifying, but in the absence of a moral or historical context, Then doesn't quite make it beyond horror into catharsis.

Sarah Moss's Night Waking is published by Granta.

Even if you have never read a book by Julie Myerson, the chances are you will already have an opinion about the author. This is the woman who two years ago found herself at the centre of a media brouhaha over her decision to publish The Lost Child, which unsparingly charted her son Jake's transformation from a bright, academically gifted teenager into an aggressive and bullying drug user. One commentator called the book "a betrayal not just of love and intimacy, but also of motherhood itself". Another declared it "a moral failure" and accused Myerson of trading the love of her son for her own ambition. Jake, who had been kicked out of the family home, was moved to tell the Daily Mail that he felt his mother's actions were "obscene".

Whatever the rights or wrongs of what Myerson did, the controversy still trails her like a swarm of bees and the quality of her prose tends to get overlooked as a result. Myerson, who writes both fiction and nonfiction, has never fitted easily into any category. Her novels are too literary to be straightforwardly commercial and although she writes with an acute eye about families, the domestic detail is never the humorously cosy kind that slips into marketable chick-lit territory.

There will, I imagine, be plenty of people who avoid reading Then, her latest novel, because of a preconceived notion of the woman who wrote it. They would be wrong to do so. Then is a bold, uncompromising book written with a deftness of touch that marks out Myerson as a truly interesting and risk-taking author.

The opening chapters, set in a post-apocalyptic London beset by violence and confusion, make few concessions to the reader. Instead, Myerson plunges us straight into the first-person narration of a woman who has lost her memory and who is living in an abandoned office building in the heart of the city. We are not sure what has happened, who she is or whether to trust what she is saying  frequently, her version of events is challenged by a handful of other survivors: Graham, a former city lawyer; teenagers Ted and Sophy; and a shifting, slightly sinister adolescent boy called Matt.

The narrator, whose name is not revealed until two-thirds of the way through, is reliant on this rag-tag group for her continued survival, but it is an uneasy alliance, fraught with tension, mutual irritation and sporadic sexual aggression, never explicitly described but menacingly hinted at: "I wake in the night. His leg nudging mine. His hand on me. His breath in my ear I don't bother saying anything. Instead, I lick my lips and I try to taste what is around me. The salty darkness. The silence above us and below."

While the narrative of the present-day nightmare unspools, the protagonist's mysterious past life is gradually revealed in a series of non-linear flashbacks. As she begins to make sense of what has happened, the novel flicks back and forth between glimpses of a family life lived along normal lines (with a dog, a spacious house, a garden with fish in the pond) and the harrowing present day (where dead babies lie stuck underneath frozen puddles on the street). And yet, the quality of her remembrance is dreamlike and uncertain so that the reader is never entirely sure what is real and what is not.

Myerson's cleverness lies in her supremely confident ability to play with structure, drip-feeding us snippets of memory yet never losing pace along the way. As the narrator struggles to reconstruct, so do we. In many respects, Then is a thriller, but one in which the villain is not one person but the bleak and unfamiliar city where nothing can be taken for granted and every day is a battle to survive. The careful, intriguing plotting ensures that the eventual conclusion is both disturbing and moving.

Myerson's use of language, too, is striking: her sentences are sparsely poetic and she is able to convey layers of meaning through the use of a single, telling detail which juxtaposes the mundane with the inconceivable. When the narrator walks around the abandoned building, she finds herself imagining its previous occupants: "The core of an apple, dampening the sports section of the newspaper it has been placed on A pair of trainers, socks stuck in them, waiting to be run home in." Much of what Myerson is dealing with here is uncompromisingly dark but the prose never feels overcooked. When a young girl dies: "All she has on now is a dirty pink T-shirt, specked all over with her blood and gone slack at the neck to reveal the pale jag of her collarbone."

If there is a criticism, it is perhaps that the protagonist is sometimes, by necessity, too vaguely drawn to be wholly likable. She makes difficult, occasionally brutal choices that are driven by grief and denial. Although these can be understood in the context of the post-apocalyptic nightmare she finds herself living through, they are sometimes hard to empathise with.

Of course, characters do not need to be likable to be convincing and Myerson just about manages to inject enough human warmth into the latter half of the plot to balance out the book's light and shade. Then is not an easy read, but it is a grippingly good one.