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.