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.

If you have one nearby then maybe, but I bet you even they don’t want that slightly water damaged, smelly copy of some cheapo 80s encyclopedia.

Around here it’s also relatively common to sell old books by weight/volume, either on flea markets or classifieds/Ebay. But sometimes it literally isn’t worth the effort.

My local thrifty shop had a pallet of assorted books and you can fill a bag for $5. I think of it like a lazy person’s library, I get a bunch of books, pay my dues, and then return them when I’m done.