Programming in Objective-C 2.0 (book, $44.99)
This is the latest release of Stephen Kochan's series which some consider to be the Objective-C bible. It has a wealth of information jammed into almost 600 pages, and it will take you from simple variable assignments to advanced class implementation. I'm over halfway through this book, and the text is easy to read (not too high level), and is broken up in a visually appealing style with sufficient whitespace to be gentle on your eyes. If you're only interested in programming for the iPhone, you may only want to rely on this book for its wealth of foundational Objective-C material as it only has one chapter devoted to the iPhone.