For Android-based programmers, we will be making direct use of two books: The Busy Coder’s Guide to Android Development, and The Busy Coder’s Guide to Advanced Android Development. It is part of a set of three books that can be purchased online for $USD 40. See the Commonsware web page for all three books. In addition, the third book is a series of tutorials on specific topics: Android Programming Tutorials.
If you're going to use the iPhone, I'd recommend Beginning iOS 5 Development, by David Mark, Jack Nutting and Jeff LaMarche, which can be purchased electronically, right off the Apress website.