A book from the Effective Software Dev Series. Written by Matt Galloway
Effective Objective-C 2.0 is a book for iOS and Mac OS X developers who wish to further their knowledge of the language they use. It forms part of the Effective Software Development Series, in which books comprise a number of Items, each of which is a bite sized chunk of information that can be read individually.
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This is absolutely the best guide to ObjC / Cocoa so far, and I own more than a dozen of the most popular books on the subject.
A concise list of must knows for Objective-C. Great companion in the Effective series.