Learn Objective?C on the Mac (Learn Series)

This book takes an introductory look at objective-C based on the reader already knowing C programming. It does an excellent job of presenting the major features and building on top of what went before. It includes some examples using Cocoa (I can't imagine that you could otherwise). It correctly makes a major point in its treatment of memory management throughout the book and details the difference between retain-release method, memory pool method, and garbage collection (new in ObjC 2.0). It also makes note that iPhone development primarily requires use of the retain-release method.
As C is a minimalist language and Objective-C is a minimal layer on top of C, there is not a huge amount of material to cover (Almost 1/3 of the book covers ObjC 2.0.) This makes the book a quick read, but does not diminish its value. Compared to shorter treatments of Objective-C, this provides a much better explanation.