We recommend the following books to beginners and experts alike:
Cascading Style Sheets: The Definitive Guide, by Eric Meyer (O'Reilly, 2000; ISBN 1565926226).
Eric Meyer on CSS: Mastering the Language of Web Design (New Riders, 2002; ISBN 073571245X) and More Eric Meyer on CSS (New Riders, 2004; ISBN 0735714258), both by Eric Meyer.
Designing with Web Standards, by Jeffrey Zeldman (New Riders, 2003; ISBN 0735712018).
Web Standards Solutions: The Markup and Style Handbook, by Dan Cederholm (Friends of ED, 2004; ISBN 1590593812).
Special Edition: Using XHTML and HTML, by Molly Holzschlag (Que, Special Edition 2002; ISBN 0789727315).