What do you do with old reference books?

a couple of them are under my work monitor...

It's a tough question. I have a bunch of very large college textbooks and reference manuals that my professors told me would be essential, but if I want the enthalpy of steam at 400C, I can just ask Google. I have my father's old CRC of Math and Statistics from the early 60's, but Wolfram Alpha does a lot better with complex integrals than I ever did. Still, those books have some nostalgic value to me. I think I will eventually trash most of them and hand my father's CRC down to my son, but probably not until we downsize from this house.

Years ago, I had already reduced books from my university studies to standard works and topics that still interest me today.

We downsized a few weeks ago. Nearly all scientific books went to the paper collection.
On the one hand, the content was at least partially outdated, then there are newer laws (in law) or completely different research results (in history). And what's more, most of it is available online, either completely free or via my university library account.

We have put other literature in public bookcases, but a lot, really quite a lot went to the paper recycling.

Old reference books are becoming as difficult to give away as pianos.

Leave them on the book shelves to amuse the younger folks.