So, I buy a book for a kindle. Can I read it anywhere else? No. I buy a book for my ipad. Can I read it on my nook? Nope! I have shelves of books and when a friend comes over and says, "Can I borrow this?" I can say no but it's my choice. I can also say, "Yes."
But until ebooks provide the ability to share they won't be the next big thing.