The 20th century has some of the great literary classics. Some English-language authors to consider include Hemingway, Orwell, Joyce, Steinbeck, and Fitzgerald. Or, alternatively, look at some of the later 20th century authors, such as John Irving or Toni Morrison. Take a look at Random House's list of the top 100 books of the 20th century for more ideas.
Here are five key works of different genres: each author has written more you can investigate: Night, by Elie Wiesel (Holocaust; key to understanding the entire century); The Plague, by Albert Camus (existentialism); The Bluest Eye by Toni Morrison (black woman); The Lord of the Rings by Tolkien; everything by Dr. Seuss (Theodor Geisel) if you can study children's literature.
Some of the prominent 20th century writers include Ernest Hemingway, William Faulkner, F. Scott Fitzgerald, Ralph Ellison, James Joyce, Alice Walker, John Updike, Philip Roth, James Baldwin, Amy Tan, and Toni Morrison.
In american fiction I would say F. Scott Fitzsgerald would be interesting, he wrote The Great Gatsby. THis is about "rich" people and those that achieved wealth. Steinbeck wrote the Grapes of Wrath which is about poor families trying to hang on during the great depression, also Faulner and Hemingway, check them out.