The best book I've found for picking up rails was the latest edition of the Agile Web Development book, which I actually own 3 different editions of it, for differing versions of rails.

The lack of a Django book, probably has more to do with the fact of the community being more focused on tutorials and Blogs rather than cashing in, although there are a few django books out there. That said with the documentation and tutorials out there I've never felt a great need for a missing book.