To my surprise my local library doesn't accept book donations. They have to get them through their own buying program. Which is unfortunate, my wife wanted to donate Japanese books because they have so few of them, she even bought extras for that purpose, but they can't take them.
It's worth considering just sneaking them onto their shelves.
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).