I suggest you to do a search after iOS 5 on Amazon and check the comments before buying a book. I'd stay away from any of the Appress books on Objective-C, iOS, etc ... low quality of information and a lot of typos.

Personally I don't have a preferred source and I tend to use Apple's documentation or tutorials from websites like:

http://www.raywenderlich.com/

iOS is a moving target so it is difficult to find a good book on this.