I bought this before teaching a theme seminar to make sure I wasn't forgetting anything that I should cover. It seems well written, but the content just misses the mark. About half the book is, more or less, "how to install contrib themes". If you're looking to learn some in depth info on building custom themes, you may have to look elsewhere.
I have not read this book from cover to cover. But I can always find the answers for many questions I need easily. The book is very well written, and also easy to grasp with the info. If you are a beginner with Themes for drupal, this is a great book.