CSS still surprises me almost daily. I'll be probably fluent in Haskell before I ever get CSS. What would really help me is having the source code of a well-written, high-level CSS rules engine - written for educational purposes, not for performance.
Also, it's free and you can keep a copy of the PDF on your computer and search it when you need something specific. It's more conversational and has more illustrations than most internet spec documents. However it doesn't tell you what parts of CSS aren't implemented correctly by specific browsers or give thorough examples.
The W3C HTML and HTTP documents are really good too:
If you're more advanced or just looking for a brush up i second Dan Cederholm's book aimdave recommended.
http://www.amazon.com/CSS-Mastery-Advanced-Standards-Solutio...
It does a great job of calling out CSS bugs in specific browser versions and describes how to work around them.