As I see it, theres already a quite useful and working system for eBooks thats like iTunes, but for books, and thats webscription.net, although there is one big difference, NO DRM. I've personally been using this website for a few years to get ebooks, It started off with only one publisher (Baen) but lately it seems to be branching out with other publishers as well (although slowly). It doesn't have many different genres of books, but it is selling current and future releases and offers large portions of the books to preview.