Tom Stovall's excellent new book (pictured above) is out via Packt's Beginners's Guide series. It's titled Drupal 7 Mobile Web Development and is the definitive guide to mobile development via Drupal. I served as technical reviewer for this title and I highly recommend it. Tom gives a solid overview of the basics including developing a responsive theme for Drupal; and also goes into more advanced topics including using Features to deploy your code. Get it while it's hot off the press!