If like me you've got shelves full of books that you enjoyed but don't think you'll read again this may be of interest:
BookHopper is the free book swapping community that lets you swap your old books with other BookHoppers.
Simply register then go to offer some of your old books to trade. When you have done this you will be able to choose from the books that other people are already offering. You won't have to wait to request your first book. All you have to do is offer three books that you have on your bookshelf that you are willing to post to other BookHoppers (offering more than three is better for everyone including yourself).
You might ask "What prevents people from just taking books and not giving any back"? The answer is that BookHopper knows how many books you have sent, received and are offering. You are only allowed to request 3 books without sending any. There is also an ebay style feedback system.
No money changes hands, and you don't have to make any payments. Simply have the good will to send other people your old books when they request them and in return you will be able to have others send you their books. The sender pays the postage (should be approx £1 uk). The BookHopper motto is "Give books to others as you would like books given to you".
Alternatively you could go down the local library.