This is one of the few reference books that I have actually sat down and read cover to cover, and it was completely worth it. For years, I had been struggling with using style sheets. I had learned HTML in a sort of mashed up, thrown-to-the-fire, just-do-it sort of way, but when I tried to do the same thing with CSS, well I failed to grasp the technology's simple beauty and produced frighteningly horrible style sheets.
Finally, I decided that enough was enough and that I had to get this CSS thing sorted out. I went to the book store, selected this book from a myriad of others, on account of its simplicity and clean layout. I then brought it home and started reading.
And the light came on. Over the course of just a few chapters, Wyke-Smith outlined the basic concept of CSS, demonstrated its wide range of uses and helped me establish a firm conceptual foundation, which has allowed me to implement complex style sheets easily and (relatively) painlessly.
If you are just learning CSS, I highly recommend this book.