I'm thinking about creating an ebook reader as a personal project, but I don't have any intention to enter ebook selling business!

I mean, I'm just considering creating a simple reader, and I want it to be useful (surprise!), so I want to be easy for users to add their books, and since most of us buy our books from Kindle or iBooks, I want to know if there's any public or private API or any other way that I could fetch bought books from the user's account, sync progress, save notes, ...?

P.S.: I know you could buy books from one of these and then pirate it, but hey, I don't want to incite users to illegal behaviour!