'The Stranger' and the 'Plague' by Albert Camus. Camus is masterful at creating complex troubled characters and their interactions in an existentialist style.
The Plague "Although Camus's approach in the book is severe, his narrator emphasizes the ideas that we ultimately have no control, irrationality of life is inevitable, and he further illustrates the human reaction towards the Ďabsurdí. The Plague represents how the world deals with the philosophical notion of the Absurd, a theory which Camus himself helped to define." wikipedia