In-app purchases

I would love to see the possibility to make in-app purchases.

By that I mean, that the app itself is free in the app-store, but provides extra content inside, that the user can purchase. It could be one free book, and 3 others that the user can buy, or perhaps small games, or even items that can be used in a game.

It's a very used concept, and for good reason.
It's hard to make people decide to buy an app they know nothing about, but it will be easier to sell them something inside an app, when they already know the app, and the quality of it.