My first reaction to this book was fairly negative after looking though the table of contents. The book doesn't cover CCK or Views theming at all - 2 essential parts of almost any current Drupal site.  I was hoping it would cover CCK and Views theming in detail.  However, when I got over the initial disappointment, I found a lot of good content in the book. It explains how themes work in detail in Drupal 5, how to modify existing themes, find additional themes for Drupal, and making themes from scratch. It even covers theming forms and making your forms use button images (something that is not so straightforward in Drupal).  
 In the introduction the author, Ric Shreves,  explains the books if for web designers and developers approaching Drupal theming for the first time. In this context, the book does a pretty good job. He says the book is not for hard core programmers.  
For me personally, I have done some Drupal theming so some of the book is not so interesting. Yet, the book still highlights some areas like theming forms  and template overrides that I need to look up when theming new sites. 
Packt has a referral program with the Drupal Association for this particular book - by using http://www.packtpub.com/drupal-5-themes/book/mid/121207qvb0pa to buy it the Association gets a little cut. Be sure to buy here if you decide to purchase a copy.
 
The verdict?   The book is work serious consideration if you are new to Drupal theming. If you are fairly experienced with themes in Drupal, look at the free download chapter and the table of contents to see if you think it offers enough value for you.  Free chapter is here: http://www.packtpub.com/drupal-5-themes/book.
 
Disclosure: the publisher sent me a review copy.