http://itunes.apple.com/app/api-reference-for-ios5/id440176532?mt=8
This is a handy reference guide for iOS5 library public API, which includes class reference and its Properties/Class Methods/Instance Methods/Constants, Above that each contain abstract/declaration/discussion.
To be able to quickly and easily find the usage anywhere, This app is well-organized and supports very smart query, All of these are Offline reading, No internet connection required.
Covers the following framework libraries:
iCloud,Twitter,Accounts,CoreImage,GLKit,CoreImage,GSS,
Foundation,AVFoundation,AddressBookUI,AssetsLibrary,
CoreData,CoreLocation,CoreMIDI,CoreMotion,CoreTelephony,
EventKit,EventKitUI,ExternalAccessory,GameKit,MapKit
MediaPlayer,MessageUI,OpenGLES,QuartzCore,QuickLook
StoreKit,UIKit,iAd