Yeah, I wish that libraries would have the funding for a "last copy" system where at least one copy of every book which was ever in their system was kept in storage --- yes, there are systems such as the Library of Congress to serve as book repositories, but they don't loan, and driving there for research is a pain.

Donate them to Better World Books or some similar group?

Donate them to free libraries or charity shops. Oxfam has bookshops and they often have old books.

If you hate them, that's all the more reason to do a destructive scan on them and upload them somewhere. If you don't care about the book, you can make a beautiful copy, and somebody wants to read it. Even if it's just to research religious fanatics around the Boulder, CO area who were in a business conflict with someone from the Jones family. They might be interesting because of who published them, because one has a list in an appendix of the businesses in downtown BFE from 1982, or because the grandson of the author became president of Chile.

I want to say that you don't know what books are worth reading until after at least 1000 years, but actually all books are worth reading after 1000 years. They become tsukumogami, somehow old and wise even if they started dumb. The list of the garbage from the 16th and 17th century I've spent all day reading that is exactly their equivalent of the self-published AI slop novel is embarrassingly long.

If there are already good pirated copies of it, cut the spines anyway and you can use the pages for fill when you're packing things. Lots of O'Reilly in my life went that way.

edit: the problem is having access to a indy print shop that will scan them for you (it only takes a few minutes, but the machine is huge and expensive.) Most of them are closed down and have been replaced with chains. I was spoiled for a while working with one.