I am using this fat book (mine is iOS5 version ) as my best reference for understanding all behind the iOS concept programming. What makes me really happy is that Matt provides in-deep look into every important aspect of iOS programming in a style that is not just ‘tutorial for monkeys’ but rather ‘full understanding’ . His book preview can be also found here : http://www.apeth.com/iOSBook/ in fact this was my original source of the book existence that lead me to purchase it in paper form. Thanks for the online version too !
samples are here: https://github.com/mattneub/Programming-iOS-Book-Examples
I hope there will be iOS7 version too.