I'm about halfway through and finding this an excellent guide to the complexities of iOS app development, Xcode and the Cocoa API. It's not really a beginner's book, though, so if you're starting iOS programming from scratch, you should work through one of the Objective-C textbooks first.
Each chapter takes you step by step through some code demonstrating a feature of iOS such as memory management, view controllers and notifications. You type in the code and read the commentary by the authors to understand what you're doing, then you tackle the "challenges" at the end of the chapters to practice what you've learnt. If you get stuck on the challenges, you can pick up tips in the Nerd Ranch forums.
Apple are frequently updating iOS, Cocoa and Xcode, so books like this need to be kept up-to-date with appropriate revisions. This edition is for iOS5. As I write, the current iOS is 6.1, and there are times when I'm working through the code examples and finding that something isn't working because Apple have changed some feature in the latest iOS. The Nerd Ranch forums are helpful for finding workarounds for these problems, though, so it's more of a slight annoyance rather than a major obstacle.
Hopefully this book will get a revision before, or soon after, the next major iOS update.