I often buy second hand books from charity shops but I'm noticing more and more places are accepting book donations and selling them for charity. There's a charity book stand in our local Tesco's and one in Wilko's too.
Our local chemist has recently set up a bookshelf for this exact purpose. The books are sold for 50p each and the money raised is donated to a local hospice.
Unfortunately, as yet, the shelves are not very well stocked so I popped back in to donate a couple of books which I've recently read. No books came home with me as there weren't any there which took my fancy, but I shall keep popping back in just in case something comes in which does.