*** 本家 ***
Objective-C プログラミング言語
https://developer.apple.com/jp/devcenter/ios/library/documentation/ObjC.pdf

Objective-C Feature Availability Index
https://developer.apple.com/Library/mac/releasenotes/ObjectiveC/ObjCAvailabilityIndex/index.html

Objective-C Runtime Programming Guide
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Introduction/Introduction.html

そのほか英語
https://developer.apple.com/library/mac/navigation/

*** 書籍 ***
荻原本 第3版／Kindle 版も有り
http://amazon.jp/dp/4797368276

HMDTダイナミック本
http://amazon.jp/dp/4861006414