This book is a long and thorough tutorial in WPF that starts right at the most basic level and adds complexity at a manageable pace. It is not optimised as a reference.
I spent my first couple of weeks trying to learn WPF by dipping in to online sources and various printed books that my new employer had. This was very frustrating because, contrary to my expectations, WPF was not just another windowing GUI. It has several complex, powerful and interacting features that make it conceptually different from the other GUI toolkits I'd worked with. Fortunately I found this book and two weeks later I am enormously more confident with WPF.
I have read criticism about lack of screenshots in this book. I can only assume that the critics have tried to use it as a cookbook. If you type in the examples or download and run the code from Petzold's website then you'll see all the pretty sample apps that you could want. Working on Vista, buttons and trees look different from how they were described in the book (which I assume was written using XP) but this caused no problems.
This book gets 4 stars rather than 5 because I found a few minor errors that don't appear in the only errata list I can find (on Petzold's own website), which has not been updated since '06. I got no reply to an email about this.
I thoroughly recommend "Applications = Code + Markup" to anyone starting to learn WPF who has the patience to work through this very long lesson.