I've been building Drupal websites for 5 years, full time for the past two. Most of that has been at the site architecture and module levels but occasionally I have been "forced" to do some theming.
Drupal 7 Theme Cookbook is now my must have companion, just like my copy of Pro Drupal Development is for my module programming.
Any Drupal book faces a daunting challenge. Drupal is so vast and flexible, it is impossible to deal with its breadth and depth in any reasonably sized book. Even limiting the scope to just theming still leaves an incredible amount to cover.
I am impressed in how much Drupal 7 Theme Cookbook covers. The early chapters can be frustratingly obvious to a seasoned Drupal developer, while I'm sure the later chapters' extensive code will be way over many designers heads.
Drupal 7 Theme Cookbook does not cover changing a site's visual character via CSS another than peripherally to other examples. It also barely glances over a few JavaScript / JQuery examples, but whole books have been written just on that topic. Drupal 7 Theme Cookbook focuses a lot of its attention to changes to how a Drupal site will look and operate using PHP and Drupal's many hooks.
Drupal can be customized using its many hooks that allow a Drupal programmer to modify how the core programming operates. Learning how to use Drupal's hooks can be frustrating the API documentation is comprehensive but only useful if you know what you are looking for. Drupal 7 Theme Cookbook does a great job introducing many Drupal hooks in the context of the goal of that recipe.
Drupal 7 Theme Cookbook is organized as a series of short recipes organized around general topics, much like a regular cook book would have a number of recipes for eggs grouped together. Drupal 7 Theme Cookbook often follows one recipe with another that expands on the task atop introduce new ideas without putting too much into one recipe.
Drupal 7 Theme Cookbook's approach to "chunking" information is great. You don't get lost trying to absorb too much at once.It does lead to repetition in order to make each recipe stand alone. Being told to clear caches every few pages starts to sound like a broken record.
Drupal 7 Theme Cookbook exposes a lot. At the same time it can be frustrating to not heave additional depth in areas of interest but again that would require a tomb that no one could carry. There are some references to additional material on Drupal.org but more would always be better.
Many books fail trying to be all things to all people. Drupal 7 Theme Cookbook does a reasonable job balancing a potential audience range from new comers to established developers. I'm glad I got my copy. I'm already putting new ideas to use in the sites I build.