The correspondence supports an Oslo court verdict last month that presumed Breivik sane, the book's author Kjetil Stormark said.

Stormark, a journalist who has already published a book on the attacks that shook the Scandinavian nation, has gone through more than 7,000 emails from four accounts used by the 33-year-old rightwing extremist before the massacre.

"The messages don't give a complete image (of Breivik), but show how patient and empathetic he seems in certain situations," Stormark told reporters.

Those traits suggest Breivik is sane and the Oslo court "made the right decision" when it found him of sound mind and sentenced him to prison rather than closed psychiatric care, he said.

On August 24, the Oslo court handed Breivik Norway's maximum prison term of 21 years, which can be extended if he is still considered a threat to society, for his Oslo bomb attack and shooting rampage on an island.

The book, entitled "The Private Emails of a Mass Killer", is based on correspondence obtained by hackers who accessed Breivik's accounts after the attacks and then gave them to Stormark so he would give them to police, which he did before writing his book.

Among the revelations are how Breivik raised money and acquired the equipment necessary for his attacks.

According to Stormark, Breivik could possibly have been detected before his massacre if Norway's intelligence service PST had a computer system capable of correlating suspicious online purchases -- such as a flashing police light and parts of the fake police uniform he bought on the internet.

The book also contains messages of support and condemnation sent to Breivik immediately after his attacks.

Breivik's lawyers had protested against the publication of the book, saying it was a violation of his right to privacy.