But then I started reading the docs. Hmm. I know a bit about relational databases. Hmm how do I do a join?
What the heck are entities and entity groups and all this new terminology. For some reason I just couldn't grock the datastore.
But then i read this book. :)
It covers both Python (primarily) and the Java version of Google app engine.
There are 4 chapters on the datastore, so you'll get a very thorough explanation of what it can and can't do. The performance impacts of implementation details etc.
There's also chapters which cover the fairly recently added task queues, XMPP, mail, and scheduled task features. All covered as you would expect from the best of the OReilly books.
So my advice, If you've read the online docs and you're having trouble wrapping your head around GAE, definitely buy it.
If you grocked the free online docs, you may still get a reasonable amount of new knowledge from this book, as there's a lot of implementation details explained, which I didn't find in the online docs last time I read through them.