A discussion titled "Books I'm ashamed I've never read" at Asimov's forum is listing some stories that may be worth reading. There are just too many in the list that I've not read.
Caution: Asimov's forum pages vanish after a while. Don't be surprised if the above link returns 404 when trying a few months from now.
A few mentions from discussion:
From Gardner Dozois:
- Theodore Sturgeon's shorts - "The Man Who Lost the Sea", "Baby Is Three", "Killdozer", "It", "Maturity", "The Other Man". Note "Baby Is Three" is probably more easily available as part of the fix-up novel "More Than Human".
- Edgar Pangborn's Davy - "perhaps the best Post-Holocaust novel ever written (with the possible exception of A CANTICLE FOR LEIBOWITZ)?" Also seconded by others. I'd not even heard the name Edgar Pangborn till now! Note: For all the praised showered on Canticle, it's a hard book to read for non-Christians. Neat concept, but a bit hard to read.
- Joe Haldeman's THE FOREVER WAR.
- 'Heinlein's "juvenalia" includes some of his very best work' but excluding PODKAYNE OF MARS & THE CAT WHO WALKS THROUGH WALLS.