Firstly I'd like to thank the provider of this letter for supporting Commander Breivik and making his prison life a little less boring.
The letter, with details of a personal / private nature removed, is available on the resource collection.
Anders Behring Breivik Letter 12-05-07
Some highlights are that the Norwegian authorities have made it a policy to declare foreigners undesirable aliens if they are pro-Breivik or aid him in one way or another. I'm not sure if there is an official term for the policy, but it's kind of like racial profiling, so we could call it political profiling, where people with undesirable political opinions are investigated and prosecuted or harassed to the full extend of the law.
In other words, Breivik has single handled turned Norway into an apartheid state where nationalists are treated as second class citizens. As was the case in South Africa this will create a totalitarian environment where armed resistance is inevitable.
Breivik explains the situation regarding his interview. He confirms that he won't appeal if he's declared sane.
Anyways, read for yourself.