Many libraries accept book donations. That way your book will be reused, possibly also providing the library with a little bit of extra funding.
Sometimes they get 'adopted', and if they don't then often whoever tries to check them out (which is presumably someone who wants to read them) is allowed to just take them as they are non-stock. And at the worst, the librarians will find another home for them (they are not usually destroyed; sometimes they join the book sale which benefits the library anyway).