When the lurid details of the Josef Fritzl case emerged in 2008, most people reacted with a sense of general shock. How could a father lock away his daughter in a basement, have seven children with her and keep those terrible acts a secret for 24 years? The novelist Emma Donoghue can pinpoint exactly how it made her feel. Just one of the many appalling details was that three of the seven children had been held captive with their mother. One, Felix, was just five when he was released. Donoghue looked at her own son, four at the time, and something clicked. "Imagine," she says now, "realising that the world is not the locked-up room you thought it was. This idea of a wide-eyed child seeing the world completely anew just gripped me. I don't know whether I would have felt that way if I hadn't been a mother of a young boy myself."
It was the seed for one of this year's very best books. Room is told through the eyes of its five-year-old narrator, Jack, imprisoned in a 3.5m-square shed with the mother he calls Ma. The mood swings of their captor, Old Nick, dictate what food they eat and which toys he plays with. Through Jack's innocent view of the world, it becomes clear that his mother has been creatively economical with the truth to protect him. Everything he sees on television - one of their only pleasures - he believes takes place in a fantasy world. Heartbreakingly, he plays a daily "game" where he has to make as much noise as possible, not realising that it's actually a cry for outside help.
But as horrifying as this scenario might be for us - and Donoghue admits that it's an almost unbearable premise - this shed is not a prison for Jack, because he knows of nothing else. He makes his mother laugh. He plays improvised 10-pin bowling with vitamin bottles. He writes a letter to Spongebob Squarepants. In fact, Room isn't the gruelling novel it might have been at all; instead, it's a story about resilience, survival and most importantly, motherhood. It's also brilliantly written, compulsively readable, surprisingly funny, and, in one key section, genuinely thrilling. Without doubt Donoghue deserved her Booker Prize shortlisting. Room has been the best-selling of all the nominees, too, which has had a rather unpalatable side effect: the suggestion that some have been drawn to the book because of a ghoulish interest in Fritzl. When it was first published in July, Donoghue admitted to being horrified that people thought she was in some way capitalising on the interest in the Fritzl case, or even writing a thinly disguised biography of the Austrian family. Now that the Booker fuss has died down, was she naive to think people wouldn't make those connections?
"I openly acknowledged that the Fritzl case was a trigger," she says, finally back at home in Canada after five months of promotion. "I had to - it would have been impossible to hide it. But that didn't mean I was setting out to write about those specific people.
"Look, if you write a novel that's really closely based on someone's life then, fine, you must expect people to talk about it, disagree with you and take you to task. But with Room, that just isn't the case. It was basically a one-line idea - a child grows up in a locked room - and then I took it in the direction I wanted to create a story. I wasn't endlessly researching the Fritzls."
Room does, though, raise the ethical question of whether it's right to base a novel that was clearly going to be a publishing sensation (the advance, Donoghue admits, was "mortifyingly huge") on someone else's famously tragic tale, even if the details do end up being different.
In the wrong hands it can seem exploitative, and, to put it kindly, none of Donoghue's previous six books have caused anything more than ripples in the publishing pond. Can her conscience be completely clear?
"Oh absolutely," she counters. "Firstly, I personally don't think it's that closely connected. But it's important to understand why and how I've written this. If Room was fundamentally a sleazy thriller then I think that's different, but you can tell from the first page that it's not going to be exploitative or voyeuristic. It's not a sordid true crime tale at all - it's a serious and sensitive book."
Donoghue is right in that sense. Room is not a crime thriller about kidnapping. It's not even a novel about what compels a monster to keeps a young woman and child locked away for years - Old Nick plays absolutely no part in the second half of the book. Instead, Room is very much concerned with motherhood and its limits. There is a really quite painful moment when Ma realises that Jack needs things she can't provide.
"I was really interested in this idea that motherhood at its best is not about holding tight but letting go - and finding the right moment to let go. Being a new mother myself was a real shock to the system, I felt it asked everything of me. And it led me to some pretty serious questions about what parents owe their children. You need to make sacrifices, of course, but are they supposed to be unlimited? So with Ma I was asking whether she has to give and give forever, or whether she was allowed to collapse into being weak again. Some readers are very cross with what she does in the second half of the book. But I'm full of compassion - you can manage that superhero parenting for a while, but not forever. I never wanted it to be a sentimental celebration of motherhood."
"I didn't want them to be seen as freaks and victims," she says. "I came to think of them like a tribe, with their own language and culture and rituals. From the outside it might seem tragic that they're standing on a table to scream every day. But it's all part of the culture they've created. In some ways it's like a buddy movie, you know. They're yoked together by the situation. There's irritation and they pull different ways, but they need their combined strength to survive. Even though it's a nightmare for Ma to raise a child in Room, he's what gets her out of bed in the morning."
In the end, it's Jack's voice that makes Room work. Novels with child narrators are unquestionably an acquired taste, but the simple language not only beautifully reflects the simplicity of their lives in an 3.5m-square shed, but provides an entry point into incredibly dark subject matter.
"I also think that protects the reader, too," she says. "Jack is fundamentally cheerful. His odd ways of looking at the world make the whole journey more bearable."
And that's the key to Room. It's not an exploitative book seeking to get to the heart of the Fritzl case. It's not a story of a neglected, abused child. It's a novel about a mother and her child.
"I'm so glad you can see that," sighs Donoghue. "Even though bad things happen to Jack, he's always got his Ma right there beside him. They are everything to each other. And her love is just unbreakable."
Room (Picador) is out now