where are the free books the kindle app offers?
The majority of books on sites like Amazon, Barnes &amp; Nobles, and the like, are definitly payed for. All of them offer some free books to get you in, but you end up in a "closed ecosystem": you can read them only with their dedicated readers, as they often use their own formats (Amazon) or at least some DRM.

If you want to go for free content, you're much better of chosing an eBook reader app supporting the open ePub format (some exemples include, but are not restricted to: Moon+ Reader, FBReader, and Aldiko. At least the first two already ship with a load of free libraries pre-configured, which they can access directly from inside the app.