This book helps you understand the logic behind the theming system employed in Drupal, and how to make it work for you. You will learn key concepts and work through examples with careful, step-by-step instructions. The main requirements to make use of this book are knowledge of HTML, CSS, and a touch of creativity - you don't need to know anything about theming in Drupal, although you should be familiar with the basic operation of the Drupal system. Although this book aims to make Drupal theming accessible to designers, theming in Drupal 6 involves writing some PHP code, and a basic knowledge of PHP will be helpful. Regardless of your technical skills, this book will teach you to design themes for your Drupal websites in the easiest way.
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.