There's a question we should ask before we proceed, an issue we should clear up.

One Of Us by Asne Seierstad, the author of The Bookseller Of Kabul, is a book about Anders Breivik and his massacre of 77 people in Norway in 2011. It tells the story of his life, and the lives of some of his victims; it also relates, in queasy detail, his preparations for the bomb he detonated outside government buildings in Oslo and his murderous attack at a youth camp on the island of Utoya. But the question is: should Seierstad have written the book, and should we be reading it?

The answer depends on what kind of response you had to the media coverage that surrounded the Breivik massacre. From the start, it was always part of Breivik's plan to attract media attention for his beliefs and as soon as the killings started, he got it. There was 24-hour rolling news, there were interviews, think pieces, more interviews and then there was the coverage of the trial itself during which Breivik read out his manifesto against what he saw as the Islamisation of Western culture. He also looked straight into the camera and gave a clenched-fist fascist salute.

With all this coverage already, it is only natural to worry about more of the same and whether the dark brand of celebrity it bestows on killers could encourage more attacks. Three years ago, during Breivik's trial, I spoke to Lionel Shriver, whose novel We Need To Talk About Kevin is about a mass killing, and she said she was concerned that handing publicity to extremists encourages more extremism. "They want to be important," she said, "they want to be noticed, but most of all they want people talking about them and they want their opinions broadcast. With Breivik it's working a treat."

But there's a problem with that approach: if you don't talk openly about terrorist attacks, what do you do? Any kind of ban, or restriction, on covering terror attacks is not only impractical, it runs counter to the obvious need of many victims and observers to talk about what they have experienced. Breivik's father Jens has published a book about the massacre, but Seierstad also outlines in One Of Us the co-operation she had from many of those affected by Breivik. They shared their stories with her, their thoughts and feelings. Many have also welcomed Seierstad's book. There seems to be a need among them to record, remember, and reflect, and in writing the book Seierstad is responding to this.

But what about us? Where do we come in, as readers? Naturally perhaps, there is a ghoulish element to it, a voyeuristic thread, a desire, or instinct, to get close to something horrible and gawp, and the book provides plenty of material that meets that uncomfortable urge; it includes detailed information about what happened to many of the victims, including Simon Saebo and Bano Rashid, reconstructed by Seierstad using interviews and first-person accounts.

It is drama documentary in book form, written with a novelistic flourish.

The book also achieves something else, which is to recognise our need, in the aftermath of attacks such as Breivik's, to explore why it happened. There can be no definitive explanation of Breivik, only clues, fragments and suggestions, but in exploring his childhood, Seierstad uncovers what have come to be considered classic signs of a potential serial killer.

First, there is his cruelty to animals. As a child, he liked to drop bumblebees in water and watch them drown. He also had what some would consider a strange relationship with his mother, giving her a sex toy as a gift and regularly asking her whether she had used it. There are other worries: his interest in certain types of hip hop and an obsession with computer gaming, both of which have been blamed for leading young men down the wrong path.

None of these factors can explain why a man kills 77 people, and Seierstad doesn't offer them up as such, but gathered together like this in the book, they might at least be symptoms of a dysfunctional personality. There are also interesting suggestions of the kind of stresses and strains Breivik might have been feeling. Some of his friends, for example, thought he was in the closet about his sexuality and should come out as gay. Some of his other friends rejected him. Then again, in many ways he was average, very average in fact. There are lots of people who hide their sexuality, listen to hip hop, play war games and as children pulled the wings off insects, and they do not do what Breivik did.

Taken together, the stories from Breivik's childhood form the first part of what is essentially a book in three sections: the first is Breivik's childhood, the second is the day of the attacks, and the third is the story of his trial. Seierstad also reconstructs the police operation that was supposed to swing into action in the event of a terrorist incident but didn't, and this material should be a warning to the authorities everywhere to plan thoroughly for the worst.

However, the main function of Seierstad's book isn't to warn of the danger of terrorist attacks, it's to remind us of the gap between us and Breivik. Towards the end she takes us to a cottage which the family of victim Simon Saebo built in his honour and lists all the help and advice offered by friends, family, neighbours and strangers. It is an appropriate way to wind up One Of Us because it puts any concern we might have about whether we should be reading it into perspective. We read about Breivik because he is different, rare, freakish, abnormal. The norm is made up of those friends and neighbours who helped build a little cabin in the Norwegian countryside, the cabin named in honour of a man called Simon.

One of Us: The Story Of Anders Breivik And The Massacre In Norway, by Asne Seierstad, is published by Virago, £16.99