The App Store Review Guidelines [1] state pretty clearly:
11.13
Apps can read or play approved content (magazines, newspapers, books, audio, music, video) that is sold outside of the app, for which Apple will not receive any portion of the revenues, provided that the same content is also offered in the app using IAP at the same price or less than it is offered outside the app. This applies to both purchased content and subscriptions.
11.14 Apps that link to external mechanisms for purchasing content 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
11.13 Apps can read or play approved content (magazines, newspapers, books, audio, music, video) that is sold outside of the app, for which Apple will not receive any portion of the revenues, provided that the same content is also offered in the app using IAP at the same price or less than it is offered outside the app. This applies to both purchased content and subscriptions.
11.14 Apps that link to external mechanisms for purchasing content 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
[1] https://developer.apple.com/appstore/resources/approval/guid...