For those with an ear to the ground, the rumblings about Room, Emma Donoghue’s latest book, have been audible for months. First came the bidding war, eventually won in the UK by Picador; then the rumours, rare these days, of an astronomical advance (the figure of €1m has been mentioned; Donoghue allows only that it was “mortifyingly large”). And at the end of last month, a fortnight before it was due to appear in bookshops, Room was longlisted for the Man Booker prize . At that point, the rumblings turned into a roar.

Until now, Donoghue’s reputation had been founded on her knack for spotting historical rough diamonds and buffing them into glowing narratives. Slammerkin, her unlikely bestseller in 2000, was spun out of a murder on the Welsh borders in 1763, while in 2006 The Sealed Letter took a notorious Victorian divorce as its grist. In the run-up to publication, however, word was that Donoghue’s seventh novel would be based on the modern-day case of Josef Fritzl, who locked his daughter, Elisabeth, in a basement for 24 years, raped her repeatedly and fathered her seven children - three of whom he imprisoned with her. Unsurprisingly, accusations of cynicism and sensationalism abounded. When I meet Donoghue, halfway through a publication tour that has mushroomed thanks to her longlisting, she recalls the period as “quite painful. A lot of people made out I was writing this sinister, money-making book to exploit the grief of victims. I was thinking, it’s not like that, but no one will know until they read it.”

She is keen, too, to contextualise the link between her novel and the Fritzl case. “To say Room is based on the Fritzl case is too strong,” she says firmly. “I’d say it was triggered by it. The newspaper reports of Felix Fritzl [Elisabeth’s son], aged five, emerging into a world he didn’t know about, put the idea into my head. That notion of the wide-eyed child emerging into the world like a Martian coming to Earth: it seized me.”

The whump Donoghue experienced on hearing Felix Fritzl’s story may have had something to do with the fact that her own son was four at the time. Donoghue has two children - Finn, now six, and Una, three - with her female partner Chris Roulston, a professor of women’s studies at the University of Western Ontario. The couple live in Canada, though Donoghue hails from Ireland; she is the daughter of renowned academic and TS Eliot scholar Denis Donoghue. Born in Dublin in 1969, the youngest of eight, Donoghue was the only member of her brood to follow her father into a literary career. She left Ireland in her 20s to complete a doctorate at Cambridge, published her first novel, Stir Fry, in 1994 at the age of 25, and has not looked back. Much has been made of Donoghue’s status as an outsider on the Booker longlist, someone who is finally getting her moment in the sun; Donoghue doesn’t view it that way at all. “I’ve been writing full-time since I was 23,” she says. “I’ve always thought of myself as a huge success!”

Her own crowded childhood could hardly be further removed from the experience of Room’s five-year-old narrator, Jack, but it is through him that Donoghue explodes any doubts her detractors might have had about the wisdom or value of her project. Living with his Ma in an 11ft x 11ft shed, knowing nothing of the outside world beyond the fantasies of the television screen, Jack is a warped version of Maurice Sendak’s Max, from Where The Wild Things Are: a boy for whom “the walls became the world all around”. His material needs are met by “Old Nick”, who comes at night bringing food and “Sundaytreat” (painkillers, new clothes), and making the bedsprings creak.

But while for us (and Ma) such an existence is horrifying, for Jack it simply is. Lacking any other frame of reference, his Room is neither small nor, in any psychological sense, a prison. Its objects, which he names as friends - Plant, Skylight, Rug - swell in our minds, too, assuming far greater proportions than the physical space would appear to allow (although in terms of feet and inches Donoghue was scrupulously naturalistic, using a home design website to ensure everything fitted). Through Jack, Donoghue pours light and air into a prison cell, and transforms his story from a prurient horror show into a redemptive tale of resilience and salvation. As I read the book, it wasn’t the Fritzl case that echoed through my head, but a couplet from John Donne’s The Good Morrow: “For love all love of other sights controls, / And makes one little room an everywhere.”

It was, furthermore, by filtering the story through Jack’s artless five-year-old obsessions (what’s for dinner? where does the poo go when you flush the toilet?) that Donoghue sidestepped any potential queasiness. “My conscience wasn’t troubled,” she says. “I knew that by sticking to the child’s-eye perspective there’d be nothing voyeuristic about it. Ma has managed to keep Jack almost oblivious to the sexual side of things - the creaking bed makes him edgy, but lots of other things, green beans, for instance, make him edgier still. I knew the chills would be justified. The book has some really serious questions to ask.”

Of all the book’s questions, those that centre on the parent-child bond are at its core. By placing Jack and Ma in a near-literal crucible, Donoghue is able to stress and test a relationship that can be stressful and testing under the easiest of circumstances. “Lots of people have called the book a celebration of mother-child love, but it’s really more of an interrogation,” says Donoghue. “I never had Ma and Jack say ‘I love you’; I thought, I’m failing if they need to say it. I wanted to conjure up that love but not have big soppy pools of it lying around. Love is what’s saving them both, yes, but there are problems to it.”

Part of the book’s pleasure derives from Donoghue’s decision not to airbrush those problems: Jack’s fizzing frustration when he senses Ma’s answers to his questions aren’t up to scratch; Ma’s flash of furious despair when Jack demands she read Dylan the Digger again. “Really, everything in Room is just a defamiliarisation of ordinary parenthood,” Donoghue agrees. “The idea was to focus on the primal drama of parenthood: the way from moment to moment you swing from comforter to tormentor, just as kids simultaneously light up our lives and drive us nuts. I was trying to capture that strange, bipolar quality of parenthood. For all that being a parent is normal statistically, it’s not normal psychologically. It produces some of the most extreme emotions you’ll ever have.”

Jack, of course, has two biological parents - but he barely glimpses the man who fathered him. Nameless and storyless, Donoghue’s Old Nick has a fairytale, bogeyman quality. Though he comes and goes under cover of dark, his presence nevertheless blankets every object in Room with a patina of threat, which Jack senses, even if he can’t understand it. “I deliberately restricted his access to the book,” Donoghue says. “I didn’t give him a childhood because I didn’t want to let him off the hook. Once he’s arrested he disappears, because I refuse to be that interested in him. As a society we’ve given disproportionate attention to the psychopaths - the average thriller is about a psychopath who wants to rape and chop up a woman. I wanted to focus on how a woman could create normal love in a box.”

Donoghue’s success in doing just that positions her book as a response of sorts to another novel based on a real-life crime. In Lionel Shriver’s Orange-prizewinning We Need to Talk About Kevin, sparked by the Columbine massacre, a mother and her son create hell in the heart of a middle-class idyll; in Room, Ma and Jack conjure humdrum beauty out of a kind of hell. “I found Shriver’s book very inspiring,” Donoghue says. “Every parent has those moments where they look at their child and think, ‘There’s a demon in those eyes and no one can see it but me!’. I could see how she extrapolated from that. With Room, I was trying to extrapolate from those moments where, as a parent, you think, ‘I’ve been stuck in this room playing with this doll for years!’ Shriver is also a great reminder that you don’t have to be a parent to write these stories [Shriver is childless]. I hate it when people say, ‘Oh, you could only have written this as a mother.’ The best book I know about being a battered wife is Roddy Doyle’s The Woman Who Walked Into Doors. Writers should be applauded for their ability to make things up.”

In Donoghue’s case, the applause has been loud and lengthy. A week after publication, Room’s commercial success (it is already the second-best seller on the Booker longlist, with only Christos Tsiolkas’s The Slap ahead of it) has been matched by uniformly laudatory reviews. Donoghue is visibly thrilled, too, by her place on the longlist.

“When I was a child, trying to get to sleep, I’d lie there thinking, ‘What’ll I wear to the Booker?’ I feel like I’ve been brushed by the feather of fame. And I see now that it’s not just about who wins, it’s about drawing attention to the business of fiction. It makes people care about books, starts an international debate about what people are looking for in the novel.

“Room,” she says, with the sort of starry grin you’d expect from someone who had just been told they’d won the thing, “has already been denounced on the Booker talkboards. You want to have that sort of passionate, angry discussion about literature. You want it to matter.”