Adding my part 
to the  meme  that  has  been  going  around, though my list surely pales in comparison. Anyhow...
Gertrude Chandler Warner, 
The Boxcar Children - First "real" book, age 5
Holy Bible (NIV) - Literal fundamentalism shaped fifteen years of my life, especially Corinthians and Ephesians.
J.R.R. Tolkien, 
The Fellowship of the Ring - First "adult" book, age 8
Orson Scott Card, 
Ender's Game - Of the vast amounts of young adult science fiction I devoured as a kid, this always stood out the most. I generally cite it as my favorite single work of fiction.
Steven Levy, 
Hackers: Heroes of the Computer Revolution - Revealed there were people in the world with a disposition much like mine.
Aldous Huxley, 
Brave New World - Had to read this for school and still muse about it fairly often.
Linus Torvalds, 
Just for Fun - I picked this up as an anti-Microsoft fanboy. Oddly, the first chapter's banter on the theory of life is what sparked my interest in philosophy.
Simon Blackburn, 
Think: A Compelling Introduction to Philosophy
John Stewart Mill, 
On Liberty and 
Utilitarianism - JSM was 
amazingly right about so many important things
Milton Friedman, 
Capitalism  and Freedom
James Madison and  Alexander Hamilton, The Federalist Papers  
Robert Wright, 
The Moral Animal, 
Nonzero, and 
The Evolution of God - Wright's three works have crystallized my world view with incisive detail. If you're going to read anything on this list, read him.