I just finished reading a book I bought one year ago. I found the book very dry at first, at least the first chapters, so I had put the book aside for many, many months... If writing CSS is a main part of what you do, you should get this book. It will confirm most of what you've acquired in knowledge through reading from a hundred different sources and from hard work over the years. But no matter how well you think you know your stuff, you will learn new extremely useful concepts and patterns.