I've worked with the App Engine for about six months and have gained most of my knowledge from the online documentation. I wish I had had this book when I started. This is the best explanation I've seen of entities and how they relate to transactions. The code examples in Python and Java are extremely helpful and give great insight into how the two languages approach the App Engine abstractions. The sections on data modeling and Django integration are particularly good. Not only did this book fill in gaps in my knowledge, but it is sufficiently detailed that I'm sure I will keep it on my desk as a reference. I've tried the other App Engine books and this is by far the best. If you have any interest in the App Engine, start here.