Woo-hoo! This is the book we've all been waiting for. Not to say that earlier books like Robert Douglass' Building Online Communities or Packt Publishing's Drupal books aren't good, but they cover a small slice of what Drupal has to offer, as well as targeting beginner to intermediate users with a range of needs.
If this book is going to be all that I think it is, it will prove an invaluable handbook on how to program with Drupal. It's clear that Drupal is becoming a framework, one that definitely needs to be on the selection list alongside things like Ruby on Rails.
If you're already a PHP programmer and you're about ready to select a framework you want to develop future web applications with, this will be the book for you.
Congrats to Matt Westgate and John VanDyk for birthing this monster. I'm really looking forward to it. Heck, I might actually learn how to program again (I trying to pick an IDE to use for Drupal/PHP development).