granny: Think we had a thread about this a while ago (smegged if I know where it is, though), and I'll say here what I said there: something like this was bound to happen eventually.
So long as people think they are buying digital goods (and given the language used on such websites (including this one) who can blame them?), they will believe they own that digital good, and can do as they always did, namely lend it friends or resell it or whatever.
Going to be interesting to see what happens.
That thread was about computer games, but it's basically the same deal. With a physical book, it's easy to know that the rights have been transferred, but with digital it's very hard to do.
Now, with systems like Steam it ought to be possible to resell and trade used games. But DRM free, is somewhat harder to get working. And ultimately, if you're not doing the trading via the original shop that sold the game, then it's not likely to work.