Derek Molloy has released a new book that is based on his popular BeagleBone book which is focused on the Raspberry Pi. I've have the book and been going through it and so far I'm impressed. It has more info than the BeagleBone book and even covers writing Linux Device Drivers on the RasPi. His web site has a bunch of goodies as well by way of videos and posts.
Here be a link to his book on Amazon:
Exploring Raspberry Pi: Interfacing to the Real World with Embedded Linux