The Liber Chaotica book (note there are three individual books, one for each power, and a big hardcover one collecting all three plus Chaos Undivided) is excellent. I don't have much interest in Warhammer Fantasy, but I found it to be a great book on it's own right, since many of the things translate easily between fantasy and 40k. It's all background with no rules stuff in it. It's really fancy, with a lot of documents dressed up like historic records, pictures, and other stuff like that. I have the heavy as hell hardback, but it's now in paperback.
Ah, and you can check out some previews here
If you want to know more about the intricacies of Chaos there is no better tome. 400+ pages of the unspeakable. Any questions, just ask. I have mine right here.
As for 40k novels, you might try the Eisenhorn trilogy, now collected in omnibus form (link
). There are also a few 40k background books.