I've found that second-hand bookstores are good for figuring out which books people treasure the most. Pick an author, scan that author's bibliography, and visit second-hand bookstores.
The works you can't find? Those are the ones people treasure.
Or, at least, the ones people won't part with for some reason.
> The works you can't find? Those are the ones people treasure.
I would say that you're mostly right, but it depends on the size and number of the print runs. If something's deemed a "classic" (frequently reprinted), or if it was a modern bestseller (mass printing), you'll find multiple copies and versions regardless of whether people treasure the book.
Despite being treasured by many people, there are tons of copies of "The Hobbit" in second-hand bookshops. 100 million copies sold will do that! [0] Yet there aren't many copies of "Tree and Leaf", and as you suggest, it's usually treasured thus not easily found. It was a special delight when I found a copy while browsing, as I'd never seen it before and haven't since, and I'm certainly not giving up my copy.
Frank Herbert's Dune, for example, has been difficult to find in second-hand bookshops (even the old mass market paperback) because there is (and has been for some time... thanks COVID) a film adaption by a prominent director (Denis Villeneuve) on the cusp of release.
The best used book stores are the ones that build their stock by buying the libraries of retired/deceased scholars. The few that I used to know went out of business, so I don't know what happens to good collections now.
The works you can't find? Those are the ones people treasure.
Or, at least, the ones people won't part with for some reason.