Books earlier than 2005 are obsolete.

Reasonable

• Professional CSS Christopher Schmitt, Mark Trammell, Ethan Marcotte, Todd Dominey, Dunstan Orchard, Wrox, ISBN 0764588338 , August 2005
• Bulletproof Web Design, Dan Cederholm, ISBN 0321346939, New Riders, September 2005
• Codin' for the Web, Charles Wyke-Smith, New Riders, November 2006
• Transcending CSS: The Fine Art of Web Design, Andy Clarke and Molly E. Holzschlag, New Riders, November 2006

Best Introduction

• HTML Dog:The Best-Practice Guide to XHTML and CSS, Patrick Griffiths, New Riders, November 2006
• Together with HTML Dog Website: http://www.htmldog.com/

Good on CSS design

• The Art and Science of CSS Cameron Adams, Jina Bolton, David Johnson, Steve Smith, Jonathan Snook, Sitepoint, March 2007, aimed at designing attractive websites
• The Zen of CSS Design, Dave Shea, Molly Holzschlag, New Riders, 2005
• Everything You Know About CSS Is Wrong!, Rachel Andrew and Kevin Yank, October 2008