A friend of mine is learning CSS. Like me, his standard MO when learning something new is to just look at the source and start playing around until you get it right. Mostly that works for CSS, but I found that there were some subtle points that I didn't just pick up and having a book helped. Here were two I found very useful:

The Zen of CSS Design : Visual Enlightenment for the Web (Voices That Matter) by Dave Shea and Molly E. Holzschlag, based on the CSS Zen Garden, was not necessarily useful for learning CSS (although it is that) but for the inspiration of what you can do with CSS. You can probably get the same thing from the Web site, but I'm a book person. I loved the rich color graphics and the ability to read it over a stack of pancakes at breakfast.

CSS Cookbook by Dan Cederholm and Christopher Schmitt is your work-a-day O'Reilly book with lots of examples, good reference material, and clear exposition. This one is handy for learning some of the subtle tricks and seeing best practice.

What about reference material? For that I prefer asking Mr. Google. If you need syntax, you can always find that online. I don't even have a favorite site--I just type css textwidth (or whatever) into Google and read the first site that pops up.