Proffesional Webdesign at an unproffesional price...Planet Productions
1) SAMS TEACH YOURSELF CSS IN 24 HOURS - Kynn Bartlett (I think).
2) ERIC MEYER ON CSS.
The first book is for beginners. it lays out the CSS in a really easy, step by step fashion. What I like about it is it talks a lot about browser compatability... which is still the achilles heel of CSS. The end of each chapter has a "browser report card" which tells you how well each rule is supported by the various browsers. Note that as browsers improve this will eventually become a moot point.
The second book takes real life page examples and "does them up" with CSS. Eric Meyer knows his stuff, and I highly recommend his book. it does NOT teach CSS, though... more HOW TO USE CSS. That is why I put this as the second book if you are a beginner.