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Following is a list of resources you can turn to for more information about the Objective-C language.
The Objective-C 2.0 Programming Language. Apple Computer, Inc., 2008—This is the best reference available on Objective-C language and is a good book for you to read after completing this one. You can get to this text either through Xcode’s Help->Documentation window or directly online from Apple’s Web site. Here is the online link for the pdf version of this text: http://developer.apple.com/documentation/Cocoa/Conceptual/ObjectiveC/ObjC.pdf.
Object-Oriented Programming: An Evolutionary Approach, Second Edition. Brad Cox and Andy Novobilski. Addison-Wesley, 1991—This is the original book about Objective-C, coauthored by Brad Cox, the designer of the language.
Objective-C Pocket Reference. Andrew M. Duncan. O’Reilly Associates Inc., 2003—This is a terse reference for the Objective-C language.