What is the best CSS book?

The 2nd edition of this (wonderful) book is due out in August.

you can't go wrong with CSS Mastery. It's also concise and very well written.

Seconded. Instead of the usual dry "Chapter n: Divs, Chapter n+1: Links" etc, Zen of CSS looks at a number of stunning designs in turn, explaining how each was achieved. Beautiful book.

I also own zen of css design, which is pretty good if you must have a physical book.

The Zen of CSS Design for me.

Bulletproof Web Design is the only CSS book I have liked enough to buy so far.

Not that the Definitive Guide isn't also a very good book.

I really like this book and highly recommmend it.

That book goes hand in hand with Eric Meyer on CSS.

Eric myer on CSS

CSS Mastery for me is THE book on CSS.

CSS: The Missing Manual was a great starting point for me.

The definitive book is Designing with Web Standards.

HTML with CSS and XHTML by Elisabeth Freeman & Eric Freeman seemed like a breeze to get through, and very easy to understand. I recommend starting here.

Trascending CSS

I also recommend Web Standards Solutions: The Markup and Style Handbook