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There are a number of books about Cocoa or the Mac that are relevant to the iPhone. Most of these books will help you deepen your understanding of particular topics. Whereas this book covers how Cocoa works, giving you a firm grasp of the how Cocoa is designed, Erica Sadun’s book (included in the following list) provides lots of sample code and serves as a good reminder of how specific classes are used.
Apple Computer. 2009. The Objective-C 2.0 Programming Language. Cupertino, California: Apple Computer
Dalrymple, Mark, and Aaron Hillegass. 2005. Advanced Mac OS X Programming. Atlanta, Georgia: Big Nerd Ranch Inc.
Dudney, Bill. 2008. Core Animation for Mac OS X and the iPhone. Raleigh, North Carolina: Pragmatic Programmers
Gelphman, David, and Bunny Laden. 2005. Programming with Quartz: 2D and PDF Graphics in Mac OS X. San Francisco: Morgan Kaufmann
Miller, Greg. “Exploring Leopard with Dtrace,” MacTech magazine, Volume 23 (2007)
Sadun, Erica. 2008. The iPhone Developer’s Cookbook: Building Applications with the iPhone SDK. Boston: Addison-Wesley Professional.
Singh, Amit. 2006. Mac OS X Internals: A Systems Approach. Boston: Addison-Wesley
Zarra, Marcus S. 2009. Core Data: Apple’s API for Persisting Data under Mac OS X, Raleigh, North Carolina: Pragmatic Programmers
Zdziarski, Jonathan. 2008. IPhone Open Application Development, Sebastopol, California: O’Reilly