Tough call...can't realy name just one of course. I'll toss out some supreme examples (these may be literary masterpieces, mind stretchers, etc)
The Real Story - Stephen R Donaldson
Illusions - Richard Bach
Lies My Teacher Told Me - James Loewen
Hero With a Thousand Faces - Joeseph Campbell
The Bible
The Qur'an
The Lord of the Rings - JRR Tolkien
The Hitchikers Guide to the Galaxy - Douglas Adams
A Buddhist Bible - Dwight Goddard
The Nag Hammadi Library
The Gateless Gate - Ekai
The Tibetan Book of the Dead
How the Mind Works - Steven Pinker
Dune - Frank Herbert
Hamlet, Macbeth, Romeo & Juliet - Shakespeare
A People's History of the United States - Howard Zinn
The Commanding Heights - Daniel Yergin
Psychology and Religion - Carl Jung
The Marriage of Sense and Soul: Integrating Science and Religion - Ken Wilber
Personal Knowledge - Michael Polanyi
In Justification of Faith - William James
An Attack Upon Christendom - Soren Kierkegaard
Thus Spoke Zarathustra - Fredrich Nietzsche
A Very Easy Death - Simone de Beauvoir
The Sociology of Religion - Max Weber
A History of God - Karen Armstrong
Beyond Freedom and Dignity - B.F. Skinner
Towards a Psychology of Being - Abraham Maslow
Forms of Intellectual and Ethical Development in the College Years - William Perry
The Atman Project - Ken Wilber
Toys and Reason - Erik Erikson
Insight and Responsibility - Erik Erikson
Archetypes and the Collective Unconscious - Carl Jung
The Development of Personality - Carl Jung
Cognitive Development - John Flavell
The Essential Piaget - Jean Piaget
Moral Development and Moral Education - Sara Sanborn
In a Different Voice - Carol Gilligan
Moral Development; a Guide to Piaget and Kohlberg - Ronald Duska & Mariellen Whelan
The Philosophy of Moral Development: Moral Stages and the Idea of Justice - Lawrence Kohlberg
The Evolving Self - Robert Kegan
Civil Disobedience - Henry David Thoreau
United States Constitution & Declaration of Independence
Starship Troopers - Robert Heinlein
I could list another 500 that are basic required reading to gain any real understanding or insight into life, but the above are a fairly good starting point...with some entertainment thrown in on top.