Browsing the young adult fiction aisles in a bookshop, you probably wouldn't expect to find a story about terminal cancer. And yet that is where, alongside the vampires and deadly duels, you will find Patrick Ness's award-winning book, <em>A Monster Calls</em>, about a young boy whose mother is dying. Ness's novel, which has been praised for its handling of a difficult subject in an honest and unsentimental way, was the first book to be awarded both the Carnegie Medal and the Kate Greenaway Medal for Illustration - Britain's two most prestigious awards for children's fiction - earlier this year. Not only that, it received a glowing review in <em>The Lancet</em>, the renowned medical journal, in which a children's book review is a rare occurrence. The story follows the 13-year-old Conor O'Malley, a boy who is struggling to deal with the terrible truth of his mother's illness. The word "cancer" is never mentioned, though from the descriptions and the fallout of her chemotherapy, we are left in no doubt. With his father living abroad, a grandmother who is drowning in her own pain and teachers who tiptoe around him, O'Malley has no one to turn to and no one who will be straight with him about the inevitable. His nightmares take the shape of a monster, which visits him at night and tells him stories that help him to process his emotions. The book's origins are equally intriguing. The idea was that of Siobhan Dowd, the Carnegie-winning children's author, who died of breast cancer in 2007 at age 47 and started writing the book towards the end of her illness. "There were some of the characters," says Ness, "the beginning and the structural idea, so not a tonne, but what was there was, I would say, really potent." The meat of it, though, is Ness's. It is a finely judged and deeply moving book (several parts made me cry), with fantastically dark illustrations by Jim Kay. "This is a book readers will remember and return to over and over again," said Rachel Levy, the chairperson of the Carnegie Medal judging panel of 12 librarians. "It is, quite simply, one of the defining books of its generation." So how did Ness manage to portray the crushing weight of loss so effectively? "It's a book about loss," says Ness, "but it's also a book about fear of loss. Yes his mother is ill and he's probably going to lose her and that's a tough thing to deal with, but what is equally tough is that he's living with this fear that is constant and ongoing and he's lying awake thinking about it and he wants that to stop. And I think that's a universal feeling, that fear of loss." We feel O'Malley's pain so vividly because it is true and Ness never tries to shy away from the inevitable. "When I was a teenager, I never felt like books were telling me the whole truth," he says. "I thought they were telling me what the truth should be, which is a different thing. I just wanted to give some credit to my intelligence - and that is more or less Conor's dilemma. He has completely figured it out, but everyone is trying to protect him because they think he's too young. So he's in the worst stage of all. The result is that he's left to face it by himself, which is the worst possible result. "If you read what teenagers write, it's incredibly dark, so they're thinking about this and if I don't tell the truth about that then I'm leaving them to face that alone and that, to me, seems irresponsible. Sometimes mothers get ill and sometimes, very sadly, they die. And if you pretend they don't in the story, then what happens to the boy or girl who it really happens to? And so when you tell the truth about that then I'm implying that it's terrible, it really is, it's the worst thing that will ever happen to you, but you'll survive, then hopefully that hope feels more true because I haven't lied about what's difficult." Ness has already had huge success with his young adult fantasy trilogy <em>Chaos Walking</em>, for which he also won the Carnegie Medal last year for the final instalment, <em>Monsters of Men</em>. How has he managed to speak so convincingly to his young adult audience? "When I'm writing for teenagers I'm trying to write for the teenage me," he says. "I never understand how so many adults have so much trouble remembering what it's like to be a teenager. I just try to keep in mind the good and bad - because there were good bits, too." Follow <strong>Arts & Life on Twitter</strong> to keep up with all the latest news and events