I think a good recap of the historiography of the reichstag fire can be found in this review by Richard J. Evans, a fairly well respected historian of the Third Reich era here:

http://www.lrb.co.uk/v36/n09/richard-j-evans/the-conspiracists

The long and short of it is that "The Brown Book" was a well crafted conspiracy theory created by left wing authors to counteract the propaganda and conspiracy theory that the Nazi regime was concocting about the communists starting the fire.

You can think of the Brown Book as something like a primary source. You probably want to spend your $50 on a later author that puts it in historical context.