Kate Chopin, The Awakening. Written & set in the late 19th century, the story of Edna Pontellier's journey to self-awareness. (I love this)
On a similar theme, Charlotte Perkins Gilman, The Yellow Wallpaper. Again late 19thC precursor to feminist literature.
Both are Novellas so a good way to sample their work.
I think someone already mentioned Edith Wharton - Ethan Frome or The Age of Innocence for openers.
The Great Gatsby. (love this too, less keen on Tender is the Night.
The Sound And The Fury by William Faulkner. Takes time to get into first person stream of consciousness at first but worth persevering with.
Have to admit I quite enjoyed Moby Dick, but never really got into the leatherstocking tales by Fenimore Cooper.
Edgar Allen Poe is v readable and great, and another gothic tale to try alongside would be Henry James TheTurn Of The Screw.
For 20th century, John Dos Passos USA Trilogy, and Jack Kerouac On THe Road just about fits your pre-50's.
Others have mentioned Steinbeck - my personal favourite wouls be East Of Eden.
Plays - most things by Tennessee Williams and Eugene O'Neill.
Zora Neale Hurston ; Their Eyes Were Watching God. Excellent, and interesting alongside the more recent Beloved and Jazz by Tony Morrison for a different womens lit perspective to that of chopin and gilman.
There are loads and loads more but they are a few 19th and early 20thC off the top of my head - hope that helps.
Highly tempted to go and rummage through my bookcase now when I should be going to sleep 