I learned from Elizabeth Castro's HTML for the World Wide Web, 5th Edition, with XHTML and CSS. Dreadfully long title, but it's put together so well (pictures with concise and to-the-point descriptions), it's inexpensive ($22), and it's the only XHTML/CSS book I have ever used. Also, it was published in '03, so it's fairly up to date... off the top of my head, the only CSS2 spec it doesn't cover are text-shadows.