CSS

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