For some people, this is old news, but I didn't catch it until now. It turns out that Noah Gift (co-author of Python for Unix and Linux) and Mike Orr (an editor of Linux Gazette), are already writing a bloody book on Google App Engine!
It is going to get published by Manning. One really nice thing about Manning is that they gradually release their books in electronic form as they are getting written through their early access program. Anyway, they have a blog where you can follow their progress (and get chapters) - it doesn't look much for the world yet, but it's most likely worth keeping an eye on!