Tyler Cowen lists the best book each decade since the 1920's:
1930s: The Grapes of Wrath, John Steinbeck.
1940s: Farewell, My Lovely, by Raymond Chandler.
1950s: Invisible Man, by Ralph Ellison, with Kerouac’s On the Road as a runner-up.
1960s: Catch-22, by Joseph Heller, with The Bell Jar and Herzog as runners-up.
1970s: This is tough. There is Vonnegut’s Breakfast of Champions, Stephen King, and even Peter Benchley’s Jaws. I’ll opt for Benchley as a dark horse pick, note that these aren’t my favorites but rather they must be culturally central. Jonathan Livingston Seagull is another option, as this truly is an era of popular literature.
1980s: Tom Wolfe, The Bonfire of the Vanities.
1990s: The Firm, by John Grisham, or Barbara Kingsolver, The Poisonwood Bible. Maybe Brokeback Mountain.
2000s: Malcolm Gladwell, The Tipping Point.