That's a really good question.
There's so many books that age incredibly badly, and I've always been adamant that I won't get marooned in an increasingly outdated past. Just because I enjoyed something decades ago, doesn't mean it's good now.
For now the best I've got is Sturgeon's Law: 90% of everything is crud.
A combination of perspective, experience, and no longer relying on the discount tables at secondhand bookshops makes it a lot easier to pick the raisins out of the oatmeal of mediocrity that we once had to plough through longhand. Those other books were always kind of ehh, but we just read them anyway.
Or perhaps better-aging books pick more-universal themes and anxieties to work with, rather than more novel, topical approaches that are flashy at the time but quickly lose their relevance.
Perhaps people could find some examples from each pile, and we can try to draw out some commonalities.