In my electrical and computer engineering university, I volunteered at the department's library. Retiring professors were our biggest source of books (there was no allocated budget, so we could not buy new stock and relied on donations).

When we did the weeding to accept only the useful stuff, we separated the "rejects" into a request-only section in the basement and the rest we left outside in the hallway for free giveaway.

All was gone incredibly fast and we didn't have to handle anything else. Curious minds and free books pair together very well.