Apple Documentation

Read

Cocoa Fundamentals Guide: Communicating with objects

https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CocoaFundamentals/CommunicatingWithObjects/CommunicateWithObjects.html#//apple_ref/doc/uid/TP40002974-CH7-SW15

Xcode User Guide: Debug and Tune Your App (All)

http://developer.apple.com/library/ios/#documentation/ToolsLanguages/Conceptual/Xcode_User_Guide/060-Debug_and_Tune_Your_App/debug_app.html

View Controller Programming Guide (All)

http://developer.apple.com/library/ios/#featuredarticles/ViewControllerPGforiPhoneOS/ModalViewControllers/ModalViewControllers.html

Key Value Programming

http://developer.apple.com/library/ios/#documentation/Cocoa/Conceptual/KeyValueCoding/Articles/KeyValueCoding.html

Concepts in Objective C

http://developer.apple.com/library/ios/#documentation/general/conceptual/CocoaEncyclopedia/Introduction/Introduction.html

Refer

What's new in iOS

New Framework/API on each iOS version

http://developer.apple.com/library/ios/#releasenotes/General/WhatsNewIniOS/Introduction/Introduction.html#//apple_ref/doc/uid/TP40008244-SW1