With iOS 3.0 or later, you can purchase subscriptions and extra content from within an application using your iPhone, iPad, or iPod touch. Some examples of In-App Purchases are bonus game levels/maps, additional experience points, subscriptions, and recurring services.Source: http://support.apple.com/kb/ht4009
In order to complete an all-round product, I had to package it in such a way that further chapters in the 9/11 Commission Report could be added to the application. After some deliberation and research into exiting app's like Adobe's Photoshop touch, I decided it was the best method for curating additional content for my product.
Notes
In-App Purchase allows you to embed a store directly within your application. You implement In-App Purchase in your iOS application using the Store Kit framework. Store Kit connects to the App Store on your application's behalf to securely process payments from the user. Store Kit prompts the user to authorize the payment, then notifies your application so that it can provide items the user purchased.Source: http://developer.apple.com/library/mac/#documentation/NetworkingInternet/Conceptual/StoreKitGuide/Introduction/Introduction.html