[Mongoose]

Amazon.com: Transcending CSS: The Fine Art of Web Design (Voices That Matter): Andy Clarke,Molly E. Holzschlag,Aaron Gustafson,Mark Boulton: Books
Great book for those who already know their way around CSS. Wonderful insight on semantic css/markup and design heuristics

Amazon.com: The Zen of CSS Design: Visual Enlightenment for the Web (Voices That Matter): Dave Shea,Molly E. Holzschlag: Books
Serves as both a showcase of great designs and a tutorial of good styling practices.