*** Disclaimer ***
I didn’t get to completely read some of the books on this list. In fact, I went through the books to see the quality of the material and the style of the writing.
Cocoa and Objective-C: Up and Running – This may be considered outdated by some, but think it lays a solid foundation for beginning developers. He wrote ton of tutorials on Cocoa before it became cool with iOS and real contributor to Cocoa community.
Learning Objective-C 2.0: A Hands-on Guide to Objective-C for Mac and iOS Developers
Effective Objective-C 2.0: 52 Specific Ways to Improve Your iOS and OS X Programs (Effective Software Development Series
Advanced Mac OS X Programming: The Big Nerd Ranch Guide