You mean the books themselves? I was thinking just bookmarks/notes.
For example, iBooks can sync bookmarks and notes between devices, and because the unique hash is the same for the book on both devices it works. No need to store the book on their servers at all. I haven't bought book one from Apple, and yet it worked quite well for me between an iPhone and iPad when I used it. All that is really needed to be stored on the server is the bookmarks and notes themselves, plus the unique ID of the book. The two devices can do the rest on their own to match it to an actual book.