http://cocoadevcentral.com/d/learn_objectivec/
Getting started with Objective-C
http://developer.apple.com/library/mac/#referencelibrary/GettingStarted/Learning_Objective-C_A_Primer
http://cocoadevcentral.com/d/learn_objectivec/
Getting started with Cocoa
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals
About Objective-C
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC
About AppKit
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/AppArchitecture
About Blocks (closures)
http://developer.apple.com/library/mac/#documentation/cocoa/Conceptual/Blocks
About Objective-C Garbage Collection
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/GarbageCollection