Drupal 5 Theming by Ric Shreves
If you are a designer wanting to learn Drupal, this book is for you. It is written by a designer so there are no programming sections full of code. It shows you how you can customize Drupal's HTML and CSS in detail. The book has a good amount of screen shots, charts, and other visual aids that really help the right side of the brain.
The first 2 chapters are great setup chapters, so if you are new to Drupal they help you get started by telling many of the things designers need to know. There were several "ah ha" moments where concepts finally clicked because of the author's descriptions and examples.
This is also a great reference book. It lists all the CSS files and theme functions that come in Drupal core with a short sentence on their content or purpose. There is an appendix that charts out all the Drupal core CSS files giving you a visual lookup of all ID's and classes for each CSS file.
As I am not new to Drupal the organization was a little strange at first. Topics like overriding template files is covered a few times in first half with little detail. But after finishing the book the organization is more clear. The author spends time going over all the areas of Drupal theming to give you a good background then spends a few chapters modding an existing template and creating one from scratch where he goes into more detail.
The last chapter covers Drupal forms which could do with a book of their own almost. But the author covered a lot of good material to get you started.
"Drupal 5 Theming" doesn't cover CCK or Views theming. CCK and Views are contributed modules that almost every Drupal 5 site uses. It also doesn't cover any advanced theming topics. Which makes this book GREAT for the Druapl beginner, GOOD for intermediates, and FAIR for advanced themers.
Also with Drupal 6 almost here, if you are worried this book will be out of date, don't. There are several theming advances in D6 but the background and understanding this book gives you still applies. Plus D5 is here for the foreseeable future so if you are working on a project now, it will make your job a lot easier.
Ric has also written a nice article on the publishers website showing how to install Drupal themes.