How much interactivity required for book to be an app?

When is a book really an app, and vice versa? I have seen many apps that are books but with added interactivity, audio, games, etc. How much of this enhanced content is required to see the book as an app rather than an e-book? What is the deciding factor? Is it a percentage of the whole?

The lines are blurring with enhancements being easier to add today. I wish to sell content where it will sell the most copies. Apps seem to out sell books.

Is it acceptable to sell the same content in both stores—perhaps with differing enhancements? Does Apple have a policy published on this?