In App Purchases in iOS

Adding In-App Purchase to your iOS and Mac Applications:

http://developer.apple.com/library/ios/#technotes/tn2259/_index.html

In App Purchases: A Full Walkthrough

http://troybrant.net/blog/2010/01/in-app-purchases-a-full-walkthrough/

 

Introduction to In-App Purchases (iOS 5):

http://www.raywenderlich.com/2797/introduction-to-in-app-purchases

for iOS 6:

http://www.raywenderlich.com/21081/introduction-to-in-app-purchases-in-ios-6-tutorial