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The Objective-C language has a number of powerful features, and the Cocoa framework that is layered on top of the language is extensive. I’ve obviously not had the time or space in this book to cover either one in the depth it really deserves.
If you intend to continue developing for the iPhone, you should consider reading further on Objective-C, especially if you’re having difficulties with memory management or the Model-View-Controller pattern around which most iPhone application development revolves.