I'm a big fan of Erica Sadun's "iOS 5 Developers Cookbook" http://www.amazon.com/The-iOS-Developers-Cookbook-ebook/dp/B... She starts off showing you how an iOS app works from main() on up, which gave me an understanding of how some of the "magical" parts of iOS work, like nib loading. The bulk of the book is a decent cookbook that you can cherry pick from as needed.

If you've worked with other event-driven GUI frameworks in the past, check out Sadun's book.