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The W3C CSS2 Specification (W3C Cascading Style Sheets Level 2 Revision 1, CSS 2.1) is a great resource for searching the defined standards for CSS.
CSS Discuss: So you think you can just read some articles or books and be okay? Well, we all know that web development is about code that doesn't work at 1:00 AM while the rest of your team is fast asleep. You're going to need to talk to someone while scratching your head and finishing up that second pot of midnight coffee, so take your questions here, and answer someone else's question while you're waiting for your answer. The online CSS community has public and private forums, a wiki, and the free subscription gives you access to all their resources.
For people who make websites, A List Apart CSS topic list includes many articles on the subject of separating presentation from structure and behavior. There are many tips and tricks on CSS drop-down menus, layouts, pull outs, print design, and style switching. This site offers even more topics than just CSS resources.