Programming iOS 5, 2nd Edition, by Matt Neuburg
I'll start by saying this book is not for beginners. If you never wrote a program before, this book is not for you.
Programming iOS 5 (and its later edition) may very well be the book you need to read in order to develop for iOS devices. It serves as an introduction to Objective-C (which some people regard as what C++ should have been from the start), a thorough guide to the XCode IDE you'll use to develop your apps, to the Cocoa collection of frameworks and their user interface concepts. If you already program, the introduction to Objective-C, along with the recommended K&R re-reading, should make you comfortable with the language. The XCode IDE may take a little more time to get used to if you are too accustomed to other IDEs (or even a command-line plus plain editor mindset), but the book does a nice job explaining it. The same is true for the large set of Cocoa frameworks required to effectively build iOS applications (with a nice side effect of giving you some head start with MacOS development) the book covers.