There are books you get through once and know you don't want to read them again. Not necessarily because they're no good, although I suppose you hang on to the ones you love dearly. I often find books I think must belong to me on friends' bookshelves, only to discover I liked them so much I bought another copy to give away. But what do you do with those books you don't want to keep?
The Berliners tend to be very generous with their books. You'll often see little shelves in the hallways of apartment buildings, where the residents deposit books for people to help themselves. Or a similar set-up in pubs and cafés, or recently a wave of Facebook giveaways by private individuals weeding out their bookshelves. And there are plenty of used bookstores all around the city that buy and sell or just exchange. I tend to dump mine on Oxfam (but not very often). And then there's always Book Crossing, where you release books into the wild and see where they end up.
On his always entertaining Meet the Germans blog, Rory Maclean introduces the BücherboXX - a converted phone box with added shelves and benches, stocked by the neighbours and apparently very popular wherever it wanders - right now it's on Mierendorffstraße in Charlottenburg. Your chance to pick up a random-ish book right now and enjoy a spot of reading in the sunshine.