I'll choose Daniel Kahneman - Thinking, Fast and Slow (http://www.amazon.com/Thinking-Fast-Slow-Daniel-Kahneman-ebo...).
The way it changed my life was to make me actually think more about the way my mind operates, the decisions I make and the way these decisions affect my life. As a consequence, there were a few books I read later that were loosely related to this one in the way that they all refer to the way people think.
Barry Schwartz - The Paradox of Choice
Steven Pinker - How the Mind Works
Nassim Taleb - The Black Swan; and Fooled by Randomness
Leonard Mlodinov - The Drunkard's Walk (quite similar to Fooled by Randomness)
Carol Dweck - Mindset: The New Psychology of Success
Neil Postman / Andrew Postman - Amusing Ourselves to Death
Rolf Dobelli - The Art of Thinking Clearly (just started)
On my reading list now:
Quiet by Susan Cain - mentioned already
The Better Angels of Our Nature - Steven Pinker
Jared Diamond - Guns, Germs and Steel
Neal Stephenson - Snow Crash
Jared Diamond - The World Until Yesterday
Also, did not quite change my life, but very recommended:
Neal Stephenson - Anathem.
You may have to struggle through the beginning, but as soon as I understood the way the world he devised operates, I was thrilled completely.