I had to read this book for research and found it to be clear, straightforward, not too long. It covers the basic chronology of Breivik's acts of terrorism and prosecution, but it's main point is Norway's deliberations into whether he was sane or not.
Since it's at quite a journalistic level, it doesn't go very deeply into that conclusion. It was a bit low on insights, but there was one point that struck me as deeply ironic. Breivik was passionately opposed to multiculturalism. He also wanted a diagnosis of sanity in order to have his crime recognized as a political act - and most of Norway agreed with that, since it meant a harsher punishment!
In the end, Breivik got what he wanted, but purely because most of those involved were themselves prepared to be 'multicultural' in the way they approached his own extreme right-wing subculture. The two experts who found him insane were those who dismissed the relevance of culture to their diagnosis.
Conclusion: a well-written summary of what you maybe already read in the newspapers, back then.