To keep, or to give away?

Apparently the greatest number of books donated to Oxfam shops are by Dan Brown.

Does this mean that more books of his are bought, or does it mean that people don't finish them, or that once read, they're considered to be disposable? A kind of literary fast food? I keep books for diferent reasons. I'm not good at getting rid of any of them - even those I haven't enjoyed - because I love just possessing them. But I'm just about to get rid of The Girl With the Dragon Tattoo, because I couldn't finish it. So maybe keeping books I haven't finished reminds me of my own failngs (after all, one should finish a book in order to form a proper opinion, shouldn't one)? Whereas keeping books I've read but hated (eg A Line of Beauty) reminds me that I have at least some staying power. And keeping books I've never read at all, and probably never will, (the entire Pepys Diaries, for one) reminds me that all things are possible...