Not a fan of misery memoirs, even if fictional, I found nothing enticing about what the book jacket described as "a devastating account of a broken life" concerning a young woman, released from prison, whose memory is "invaded" by childhood abuse and neglect. That may be an accurate plot summary, but does not begin to do justice to this astonishingly enjoyable debut novel.

Nor should the book's fractured, unchronological and "hallucinatory" narrative technique fill any reader with dread. In fact the author is fairly conservative, splicing together only two segments of the protagonist's life in any given section (or "Act"), and only allowing greater fragmentation after we have grown au fait with the different parts. Psychoanalytical associations are enough to make this backwardness feel straightforward enough.

We are also helpfully provided with an advance "Cast List" of 24 characters, a note adding that "All other parts are played by innocent bystanders". The division between guilt and innocence is not quite as clear cut as that, though one thing that makes this book so readable (and the protagonist's life so plausibly livable) is the number of ordinarily decent, if ineffective, people scattered along the way  a loving grandfather, an appreciative writing teacher, a forgiving mother-of-victim.

Six names not on the cast list are the aliases of our heroine in the various phases of her life. So she is "Louise" in her 30s, post-prison, but "Lulu" as a little girl  one with a predilection for blacking up as "the African Queen of the Mountains of the Moon" (or rather warrior, complete with scarring on cheeks and chest). Little "Lulu" escapes into the derelict scrub behind her council house, calling it the Masai Mara. She has a "Daddy" whose violence is so familiar to the police that they bring her chocolate every time she dials 999, and a mother whose sick narcissism is just as ugly. This child chooses a day at school as her birthday treat, yet describes herself as "lucky".

As a young teenager, she becomes an escapee from Red Roofs young offender institution. The author depicts a damaged person  who watches herself in mirrors like a stranger, and defines herself as a freak (as opposed to "a normal")  but also a hardened survivor. This trust that our resourceful heroine will always survive is what allows us to take pleasure in her ingenious ways of doing so; the same pleasure, of watching a female victim turn the tables on her persecutors, which has made the Stieg Larsson trilogy so popular. There are thriller elements that add suspense to this very literary fiction. Yet, at the same time, we are given a glimpse of a happy ending (of sorts) fairly early on, and so are spared too much hard-to-swallow, heart-in-throat anxiety (or, as our heroine always puts it, that feeling of having one's "tonsils up").

In her 20s, befriended by a luridly memorable "Welsh slapper" named Gwen, "Beverley" (as she is then called) enjoys adventures with an almost London Fields quality  not comic per se, but with a certain hilarity amid the squalor. We feel complete confidence in the author by this stage. She has a perfect instinct for where to be elliptical and where precise. If the reader becomes disoriented, it is always intentional, mirroring the narrator's own physical and mental disorientation. The more crazy or chaotic the scene, such as the extraction of a horse from a piss-soaked bungalow lounge, or a casino shoot-up with the lights out, the more you appreciate Kay's precision  the single dab that conveys an entire minor character or chain of events. Kay uses her central character's evolving idiosyncrasies of language  saying "grim" for "grin", or "tends" for "pretends"  to signpost chronology, and to immerse us in the first-person perspective. These tics never obscure the prose or seem artificial. Later on, the novel's language slides into Joycean poetry, with sentences such as: "Fact I'm still alive knocks all the wind out of the death of me." Again, these lyricisms are rarely difficult to understand  a balance that few novelists attain.

Strikingly, when Louise leaves her deprived British life to travel in the Rwenzori mountains of Uganda (the real "Mountains of the Moon") it feels like entering relative safety, and a knife-fight scene on the Tanzanian border is less stressful to read than a brief flashback to her (Lulu's) family home. Throughout, the novel seems to take a selection of cliched metaphors about hardship  life as mountain climbing, as a roulette wheel, as a gilded madhouse or a prison  and turn them into fully realised episodes, each spanning several years. Elsewhere, metaphors are strung delicately across Louise's life, as when "Beverley" falls for a man she calls "The Oak Tree", echoing her childhood tree-climbing. This is only one of several love stories folded into this densely packed novel; a relationship which perhaps needed fuller handling for the reader to share Louise's emotional involvement in it. Similarly, the novel's mystery subplot, involving a posh boy named Quentin, never quite gets its payoff in terms of explication and characterisation.

Yet Mountains of the Moon does everything that novels can do, and does them in a very original way. IJ Kay (a pseudonym) is apparently a first-time author in her 50s and, if so, has achieved a kind of success against the odds just as exhilarating as her heroine's unlikely victories.

Mountains of the Moon tells the story of a woman in her 30s, recently released from prison, who finds herself a job filling doughnuts. But IJ Kay's extraordinary debut also tells the story of Lulu, aged seven, escaping the abuse of her mother's home for "Africa", a nearby wasteland, prowling with hyenas and jackals and the dangerous Sandwich Man. Then there's Catherine, aged 11, in a young offenders institution; teenage Mitten, gloriously, hopelessly in love; and Kim, a 21-year-old croupier, witness to a shocking crime, betrayed by those closest to her.

Each of these characters tells her own story in fragments the reader must piece together to gain a sense of the single, nameless woman who encompasses them all. This is a novel about one woman's struggle for existence, both physical and psychological, and in spite of the desperate subject matter it is above all a triumphant, uplifting expression of an individual's capacity to transcend the brutality and ugliness of everyday life and create something unique and magnificent.

Mountains of the Moon is not in any sense an easy read. Kay makes great demands on the reader, and apart from a handy cast list at the beginning, offers few concessions to those struggling to weave the various strands into a vaguely comprehensible whole. It is a bold decision, especially for a debut novelist, and one that I fear may deter some readers, but those who persist will be well rewarded. By the end of a second reading  and it took me two readings, I'll admit  there's a sense of having experienced something genuinely unforgettable.

The voice of the narrator is integral both to the themes of the book and to its impact on the reader. While adapting to each identity, it remains unmistakably her own and offers the reader a means of linking the fragments together. It's a wonderful voice: funny and fragile, innocent, knowing, tender and tough. I have never encountered another like it. As the narrator explains to the casually callous Gwen, "No one speaks like me in real life. If I didn't speak the way I does I wouldn't even exist." Quite so. For much of the story Lulu/Catherine/Mitten's voice is the only thing she has.

The narrator's struggle to believe she exists, to forge and sustain her own existence, makes for a moving narrative. Her mother is the first to try to stamp it out. She is incapable of recognising her daughter's existence, except in relation to her own. "'She speaks badly on purpose,' Mum says, 'just to show me up.'" On another occasion, "She feels down the back of my legs and I int even a horse. 'Dancer's legs like mine,' she says."

Lulu learns to split in two, to stand outside herself. In one powerful description of a car journey, she imagines herself outside the car: "Here I is, here I is, running, running on the verge. Now and then I look to see, case I'm still sitting in the car, looking out of the winder. I smiles. Sometimes I run close enough for one hand fingers on the glass.

I am real."

It's a survival strategy and an effective one, but it leaves a legacy. Love necessitates self-revelation, but when the glorious Velvit Gentleman asks the teenage Mitten who she is, she is unable to give him an answer.

"'So who are you?'

'Who do you want me to be?'"

The narrator's journey takes her from the "Africa" of her childhood, with its lamp-post giraffes and bulldozer elephants, to the "real life" Mountains of the Moon in Uganda, where imagination and reality collide in spectacular fashion. This is not a Cinderella story. Kay is too intelligent a writer to give us the happy ever after. But for all its devastating subject matter, Mountains of the Moon is nonetheless full of hope. It is notable how much kindness there is in it. Right at the start a station-café owner gives the narrator food and drink on learning that she's just come out of prison. And this is echoed throughout the novel; acts of kindness from random strangers, the tenderness between Lulu and her siblings, the real love of her grandparents, the fashion student who gives Kim her precious coat. There is nothing simplistic about this bold, unsettling, uplifting novel. Read it. Then read it again.

 Clare Allan's Poppy Shakespeare is published by Bloomsbury.