This is one of the oldest books on my development bookshelf that is still useful. I picked up the HTML Stylesheet Sourcebook in 1997 with the intent of learning the new fangled concept of Cascading Style Sheets and DHTML. While the subject was a bit overwhelming at first (there were a lot of differences between the implementation of CSS within Netscape 4 and IE 4), I was able to ramp up on the technology in short order, due in large part to all of the work Mr. Graham put into this book.
As I said at the top of this post, the HTML Stylesheet Sourcebook is still useful. While a lot of Web sites provide this information, and many other books (including some in my library) cover the same topic, I find myself returning to this one when I need a quick answer.