Programming GAE is a thoroughly readable guide, and provides a solid overview of this cloud platform, including Google's way of organizing databases (they changed the industry) and the many services GAE offers. While Google's online developer docs for GAE are very useful as well, the narrative structure here guides you through the concepts in a logical order, and at least for my style of learning, was a must for developing on this platform.
As with any technical book, the reader must absorb the bits he or she needs and skip the rest, but I found myself reading most of the book, including the overview of Django (which I don't use) and only skipped the sections dealing exclusively with Java, since my goal was to learn about Python on App Engine. Basically, I read this cover to cover and skipped ahead whenever I encountered lots of semicolons or XML.