Anything and everything you can get your hands on, except maybe certain selections from Black Library. Realms of Chaos and Lost and the Damned (not the Eye of Terror list, but some earlier books) are good from what I hear, all the Chaos Codices, the Eye of Terror campaign Codex, various Index Astartes on the Legions and other Chaotic threats (i.e. Renegades are in WD 302 US, Blood Brigade are in WD 292 US, etc.). Do a search about specific topics on this board and others, look through the threads, ask around if you don't find something... you might even want to look into Fantasy's Chaos fluff, there's a lot of parallels.
But honestly, IMHO once you read enough Chaos fluff (or any fluff, for that matter), it doesn't get that hard to make up your own. So, read up, boy, cause you got lots to learn before you can truly master Chaos*! ;Y
*"Master" is a subjective term, and can occasionally mean Chaos mastering you. Results may vary, side effects may include rampant mutation, an evil cackle, a desire to punch babies and kick puppy dogs, psychic powers, Spawndom, eternal damnation and/or a horrible death. The Chaos Gods are not for everyone, consult your Apothecary as to whether it's right for you.
