Cain, by Jose Saramago, is probably the most influential book I’ve ever read. It completely turned my view of religious works (even when viewed as literature and not holy writ) on its head.

Other works that have had profound influence on me:

The Cave - Saramago

The Gospel According to Jesus Christ - Saramago

Something Happened - Joseph Heller

The Plague - Albert Camus

Cottard (The Plague - Camus) and Anne Shirley (Anne of Green Gables - Lucy Maud Montgomery)