Bulletproof Webdesign (Dan Cederholm)
Bulletproof Webdesign is one of the best books about css that I ever read. The author was able to explain the most important rules of bulletproof webdesign in one book.
Each article of the book has a example of methods and an explanation why it is bad and how you can make it better.
I would suggest Bulletproof Webdesign for all, who are interested in learning of modern methods of web design.