There are some great books from Packt publishing.  These are about Drupal 6, the latest version of Drupal as of Feb 2010.

 One book is Drupal 6 Themes which can be ordered at http://www.packtpub.com/drupal-6-themes/book/mid/3103093akzv6 

If you use the link, they will send me a little money (full disclosure)

Drupal 6 Themes uncovers all the intricacies of the theming system in Drupal.  This is a complex and difficult part of Drupal and requires a lot of study.  I'll post more on how this book is once I get further into it.

 enjoy! 

- Selwyn