The only material I know of that covers the CSS in any detail would be the actual CSSOC course materials, but the on-line product documentation would be more up to date. The basic and advanced config guides on CCO provide all the information needed for most configurations. The only thing I've found that's somewhat lacking is the documentation of the scripting language, and that is not really even touched on in the class. (at least not when I took it over a year ago)
I just bought "Content Delivery Networks" by Scott Hull, ISBN# 0072190469. I can't give it much of a review since I haven't read it yet, but I do know it mentions the CSS and CSM, though I'm sure it doesn't go into any great detail. It's more of a general reference on content switching methods and theory, which is what I was looking for.
