Learning Cocoa with Objective-C by Paris Buttfield-Addison, Jonathon Manning; O'Reilly Media
I really enjoyed this book. Although it is not a comprehensive overview of everything you need to know to build apps in Objective-C, the topics that it does cover are thorough, and explained in a way that is very easy to follow. You'll find interesting details in this book that you wont see in other books. For example, it explains some little known details about how iOS applications handle memory, and building document-based apps. Other books that claim to be comprehensive don't seem to cover this stuff. There are excellent chapters on using blocks and queues, key-value coding (including working with files), networking, and other great topics.
The code samples are excellent. They are straight forward and consice, and they don't require that you've done the preceding examples, so you can easily skip around if you like.
This book is perfect for an intermediate iOS developer like myself, because it clearly explains many of the basics while also digging deeper into some more advanced features. And with all the changes that have occurred between iOS4 and iOS6, the book has been a wonderful way for me to learn about the new features of iOS6, while at the same time reviewing some of the concepts that have been around for a while.