Here are some novels (and plays) I view as must-reads -- both great works, and have informed my understanding of the human condition:

Things Fall Apart by Chinua Achebe
Great Circle by Conrad Aiken
The Ghost Road by Pat Barker
Death Comes for the Archbishop, Song of the Lark and My Antonia by Willa Cather
Lord Jim by Joseph Conrad
An American Tragedy by Theodore Dreiser
Invisible Man by Ralph Ellison
The House of Breath by William Goyen
Catch-22 by Joseph Heller
Soldier in the Great War by Mark Helprin
The Master Builder by Henrik Ibsen
Saint Joan by George Bernard Shaw
The Once and Future King by T.H. White