I graduated from a Catholic Prep School (last two years) so my list may be skewed a bit. I have made it a point to re-read a lot of the assignment reading I had in HS and University.
In no particular order by author not specific novel:
Steinbeck: If you ignore Travels With Charlie, which I loathed, hard to beat: Grapes of Wrath, Mice and Men, Cannery Row, etc. My favorite is East of Eden. It help that my family history is tied up in that part of the world.
Hemingway. I liked his novels more when I was young than I do now. I re-read most of them and found, The Short Stories of EH the most enjoyable. A semester in one book
Willa Cather. Death Comes for the Archbishop is great. It concerns the attempts of a Catholic bishop and a priest to establish a diocese in New Mexico Territory.
O Pioneers! as well.
Sinclair Lewis remains a favorite. I just re-read Main Street and Babbitt. Elmer Gantry is next.
William Faulkner would scare you to death.
Herman Melville's Moby Dick is worth another glance.
Hesse
D.H. Lawrence
Huxley
Fyodor D.
so many others.
Start with the American's and then expand.