Underrated Classics Recommendations?

I prefer old books to new ones (books that have stood the test of time), but I’m wrapping up what I consider to be classics and I’m looking for recommendations of older books that are still really solid if not a little more niche.

For reference, I’ve read lots of Heinlein, Asimov, Bradbury, PKD, Dune series, Hyperion series, etc. I’m about to get into Ringworld by Larry Niven and I consider that to be more niche. What are some other older books like it that may not be the first thing that comes to mind when you think of classics, but are groundbreaking classics nonetheless?

Edit: Lots of really great suggestions that have made my reading list for the year. Much appreciated!