First off, this book is definitely not a reference book, if you want to code up in XHTML and CSS, then you need to do the groundwork which is covered in other books. What this book does do, is show you how to practically apply these technologies in everyday situations of web design, and show patterns of commonly accepted, useful tricks and techniques.
Invaluable to the freshly converted - yes - but make sure you know your stuff or this book will leave you floating nowhere. There are no explanations, or details on XHTML or CSS, you must have a reasonably good grasp of both.
The book assumes we are here to learn the simple applications without being confusing. Thats cool, but the book also assumes you have a good working knowledge of CSS, so its simple, but not so simple. I was disappointed that there was not much depth to the examples shown, and some of the potential pitfalls were not indicated. For example, on the chapter on CSS positioning, were given a float method, but its not explained why this is not ideal or where to find more information about the related issues. That stuff would seem relevant to the readers of this book.
Anyway, i enjoyed it, it was really useful - all the applications are excellent, but be careful as you will probably get stuck without a grounding in XHTML and CSS.