OK, we went through this last year during a declutter, and then again during the pack.
This is how I decided:
If the classics were a relatively expensive publication, they stayed. Most of them were not. The only classics that, apart from a handful or less of absolute favourites, were the antique ones from the heirloom bookcase! Anything else, I figured that I could either borrow from a library if I ever got the bug, or I could hunt down a gorgeously bound old version (thinking of the MacMillan Facsmile series kind of binding).
I managed to get DP to see reason about most of his 'espionage and intrigue' titles, in that they weren't worth keeping once read. He could offer them to his brother (who DOES reread these types of books) or he could help us out by letting them go.
It was a big cull, and that's considering we have loads of boxes still to rehome into our strawbale home when it's built sometime this century.
Having also worked in publishing made is easy to decide which books I would find in a quality version later down the track if I felt the urge to re-own.
I kept my English Lit texts like the short story compilations and poetry collections, as well as critique texts and historical readers from my undergrad.
Oh, and I find the Salvos appreciate big loads of books Give someone the opportunity for a quality read they normally wouldn't get