CSS – The Definitive Guide by Eric Meyer, and Estelle Weyl.
- This book is very useful if you want to really understand how CSS works.
- There's some how-to, but more information on what the rules mean and how things work.
- It makes a great reference.
- From there you can understand more of the clever tricks you find online.
Head First HTML with CSS & xHTML by Eric Freeman, Elisabeth Robson
- This book is excellent.
- This is specifically for beginners, and uses a lot of innovative techniques for helping you understand and remember what you're learning.
CSS Mastery: Advanced Web Standards Solutions by Cameron Moll, Andy Budd and Simon Collison
- This is the best CSS book I have read.
- If you think you know a lot about CSS, this book will open eyes!