The impending Black Crusade has driven me to library, and again unleashed two of the classics, Slaves to Darkness and The Lost and the Damned. In my opinion, two of the best books written on chaos (and some of its foes). The vast amount of background information is a treat and the chaos renegade warbands offer tons of interesting material for both players and GM's.
If you have the books, or are forced to look on the internet for information, give them a peek. Great for Dark Heresy as well, obviously. Of course, being GW produce there are some inconsistencies here and there, but after all, it is chaos...But extra's as the history of the God-Emperor (most of it not even known in game), the future of Man as a psychic race and the danger of a new chaos power being born by the fall of Man are just some of the tidbits on offer.
Friedrich van Riebeeck, Navigator Pirmus, Heart of the Void