I enjoyed this book, I found the format and content useful. I'm broadly familiar with Drupal and typically in the architect/planner/site builder roles but haven't been deep into custom development of modules or themes previously but there were concepts and recipes in this book which were helpful in getting me up to speed and it did good job at reinforcing fundamental Drupal concepts. It covers basics of how themes work, creating sub-themes from core themes as well as briefly covers creating your own custom theme. It did a good job at addressing common developer tasks such as debugging, as well as leveraging panels and views. This book probably could be improved by presenting more on the availability and features of other base themes as well as considerations in choosing a base theme if you are doing so (granted there is much documentation and blogs available on the internet where you can make your own informed decision). Also I would have liked to have seen some coverage of options when considering theming for mobile devices.
The Drupal 7 Theming Cookbook appears to be an update of the Drupal 6 Theming Cookbook, with changes to make sure that code examples still work under Drupal 7. Under ordinary circumstances, this would be all that is needed. Unfortunately, theming has undergone a seismic shift, and merely updating the examples does not adequately reflect this change. In Drupal 6, the Zen theme was definitely the preferred starter theme. But mobile phones and tablets now make up a significant percentage of the devices people are using for web browsing. Ever since Ethan Marcotte published "Responsive Web Design" last year, responsive themes have become coin of the realm. The leading responsive base theme for Drupal 7 is the Omega theme. So, if you're putting in the effort to learn theming for Drupal 7, Omega is what you want to study. But the Drupal 7 Theming Cookbook still uses Zen as its example starter theme. This is especially unfortunate because other than that, this is a good introduction to Drupal theming. If the author were to issue a new revision of the book, modified to use Omega, this would be a good choice.