New York Review of Books invites readers to suggest titles for inclusion in its Classics series (link). Here's a personal selection. I haven't made exhaustive inquiry but I think some are available elsewhere, and some are entirely out of print. Most are stuff I already own and, in the cull, am choosing not to throw away--although I would like to lay my hands on an acceptably-priced set of the Sperber trilogy. A couple I have discussed earlier here.
Cash, W. J., The Mind of the South (1941)
Inimitable “appreciation” of the South at the end of the long antebellum.
Davis, Norman, ed. The Paston Letters (1963)
Family letters from the late Middle Ages.
Levy, Newman, Opera Guyed (1923)
Opera in comic verse.
Lippman, Walter, Public Opinion (1922)
Wonderful insight into the mind of the most influential journalist of his day.
Markham, Felix, Napoleon (1963)
The busiest life in short compass; a marvel of concision.
Matthews, George T. The Fugger Newsletters (1959)
Another retrieved letter cache.
Nicolson, Harold, Good Behavior (1955)
Instructive insight into a certain kind of British mind.
Sitwell, Edith, Planet and Glow-worm (1944)
Sperber, Manes, All Our Yesterdays trilogy
God's Water Carriers (1991)
The Unheeded Warning (1991)
Until My Eyes Are Closed With Shards (1994)
Autobiographical account of life in hard times.
Waugh, Evelyn, Edmund Campion: Jesuit and Martyr (1946)
Waugh as Catholic.
Westcott, Edward Noyes, David Harum (1900)
American local color.