It's not a book that covers all aspects of WPF, and it that sense some might argue that it's not a reference book.
For me, as a begginning WPF Developer it's the perfect book!
It doesn't develop an application as one follows the chapter. The examples are instead kept short, just hightlighting any current subject the author wishes to explain.
It doesn't cover code behind very much, the focus is mainly on xaml markup.
Since the "code coverage" is not as high as in other books, some advanced topics are left out.
That actually works to this book's advantage in many areas.
As a beginners's WPF book, and a xaml reference, I highly recommend it!