The authors of this book created the CSS standard . This discussion is comprehensive and detailed. Review added January 2001. If you read this book, also read about the reality of CSS in existing browsers.
Apart site is excellent.
This book is comprehensive, well organized, and well written. This is the book I reach for first. However, if you have no experience with HTML, you may find that it does not have enough screen shots. Review added February 2000. An HTML encyclopedia of sorts, this book is as good as the O'Reilly book and includes more screen shots.