Another vote for charity shops/thrift stores. It’s a good way to raise a little money for charity and for someone to get a book they might no afford at full price.
I don’t know about the rest of the world but here in the UK, supermarkets and businesses sometimes have a little bookshelf. You leave any books you don’t want and people can take them putting a few coins in a collection tin that goes to a good cause.
Another thing to do is take them to work, colleagues might well want them.