11.13 Apps that link to external mechanisms for purchases or subscriptions to be used in the app, such as a "buy" button that goes to a web site to purchase a digital book, will be rejected
11.14 Apps can read or play approved content (specifically magazines, newspapers, books, audio, music, and video) that is subscribed to or purchased outside of the app, as long as there is no button or external link in the app to purchase the approved content. Apple will not receive any portion of the revenues for approved content that is subscribed to or purchased outside of the app.
Source-engadget