CSS: The Missing Manual is a good step by step manual for anyone interested in learning CSS. The book does assume that the reader has some basic experience with HTML. However, the CSS related material teaches the reader right from the very beginning.

Those new to CSS will likely read this book from start to finish, completing the tutorials as they go. In this way, they will learn how to create internal and external style sheets; the basics of text and image formatting; and more advanced techniques for floating layouts, forms, and navigation bars. Those with some CSS experience will probably pick and choose topics as needed. However, I highly recommend reading the Improving Your CSS Habits and CSS on the Edge chapters as they contain a lot of important tidbits.