Karin Fossum and I settle in with coffee, surrounded by the expensive trimmings of Norway's premier publishing house. Her short, blonde, spiky hair and multi-studded ears suggest a forthright, almost aggressive personality, yet she has a gentle presence. Her red-rimmed eyes hint at insomnia and she talks with a consideration for her listener.

The Water's Edge, the latest instalment in her bestselling Inspector Sejer crime series, is about a child sex murder. When the semi-naked body of 10-year-old Jonas August Lowe is found lying on the shore of Lake Linde, a whole community is shattered.

For Inspector Sejer and Detective Skarre, it opens up a world of moral questions. When a second boy goes missing, panic ensues. Fossum uses the various perspectives of culprit, witness and investigator to create confusion and isolation for all concerned.

Fossum believes that even such an abhorrent crime should be treated responsibly. "I don't think people want to know anything about it," she tells me. "It's the worst crime, because it's so difficult to understand. I read a lot about it before I wrote the book, because in Norway we have a world-famous expert on this subject and he treats them. They can be helped, if they want treatment, but it is very difficult."

Part of the power of The Water's Edge is that it highlights how perpetrators are from all social strata and that "as long as adults make mistakes and as long as parents abuse, they will create new abusers". It's a harsh, unwanted truth.

Bizarrely for one of Europe's most celebrated crime writers, Fossum doesn't consider herself a great purveyor of her genre. "I'm not a good crime writer. I'm not good with plots... so I have to do something else." Her alternative is to concentrate on the yearnings of life's also-rans, and how fragile minds fracture when seclusion or routine is disturbed. This is when anomalies occur. Fossum describes it as a fascination with "the tragedy, the drama, the sadness" of such events. She is interested in "the good guy who does something evil" rather than the bogeyman. The former, she believes, is "much more frightening".

She has an atypical attitude to her sleuth, the kindly, slightly stuffy Inspector Konrad Sejer. "My detective is not very important to me," she says. "He's in the book because he has a job to do for me, but I never intend him to be a major character."

Equally, the relationship between Sejer and his assistant avoids the formulaic mentor-protégé template familiar to police procedurals. Over the past decade, the pair have become intellectual equals. "Sejer is a bit of an old-fashioned guy," admits Fossum. "So he needs this young one to see the world with fresh eyes."

Two real detectives assist Fossum on the finer points of her research, although she is careful not to confuse her stories with genuine cases, remembering that such crimes demand sensitivity.

At this point in the interview, Fossum makes something of an extraordinary admission, one that underlines her credentials. It is something that she tells me she doesn't talk about to Norwegian reporters, and she won't be pushed on details. "I have experienced a murderer among my friends," she declares. "Many, many years ago. At close range I have seen the impact of it. I knew the victim, I went to the funeral, I have been to the house, to the specific room where the killing took place, and I was stunned by it. It's such a blow."

It is the perpetually building wake of such an event that intrigues her. "This killing will be in people's minds for generations. 'My brother did this', 'My father did this', 'Do you know what my grandfather did, what my great-grandfather did so many years ago?' It never stops."

Fossum's novels are set in the rural community north of Oslo in which she lives. It seems entirely disconnected from the capital's metropolitan buzz.

"The countryside is a huge scene and when you put people on this scene, this naked scene, you see them very clearly."

She explains how the environment focuses the reader on the nature of the crime. "Let's say a murder happens in one of the apartments in one of these huge houses," she says, waving at the blocks opposite. "People on the next floor maybe didn't even know the victim. If it happens in a small village, absolutely everyone will know: 'I've seen him many times, he comes to buy his groceries in my store, he goes to church on Sunday...'."

Her next book will go further to investigate the psychology of the perpetrator. The emphasis, as ever, is on the why rather than the who. "I usually say that this book is about death. Then people say all crime stories are about death, and I say no, they're about killing, and that's something else."

It's a clarification Ibsen would have understood.