These 10 books aren't necessarily my 10 favorite books of all time, though some of them would be on that list. These are the 10 books that most fundamentally altered the way I think.
10) Tortilla Flat - John Steinbeck
I got this as a 13th birthday present from an aunt. The book was paired up with Of Mice And Men, which is much better known. I can only guess that this was done to make the book thicker, because the publisher did some study on the optimal thickness for book sales, and the relatively short Of Mice and Men couldn't be stretched into that thickness. I found Of Mice and Men to be kind of trite and not all that interesting. Tortilla Flat was another story entirely though. The book has a lot to say about how we fit in our friend groups,our motivations in life and more than anything else it really illustrates the humanity of us all. You come away from the novel caring deeply for relatively unremarkable characters that you wouldn't think twice about in real life. This book probably made me a more compassionate person.
9) The Things They Carried - Tim O'Brien
Read this at 17 and it probably shaped my views on war more than any other book. Namely that it's stupid and usually isn't necessary.
8) The Diamond Age - Neal Stephenson
I read this last year. A lot of different themes are covered here, very little of which actually has to do with science fiction. The book is mostly about relationships in all their myriad forms. The relationships we have with our friends, relatives, those in power, those "below" us, with society and with ourselves.
7) The Chronicles of Narnia - C. S. Lewis
Like a lot of kids I read them when I was 12. I don't think I can really say exactly how they influenced my thinking, just that they have.
6) The Man Who Knew Too Much: Steven Turing and the Invention of the Computer - David Leavitt
I don't think this book is particularly well written, but the story of Turing is enough to drive just about any biography. Alan Turing did more to defeat Hitler and save his country (Britain) than any other single person in the whole world. More than Eisenhower, more than anybody in Band of Brothers. Turing nearly singlehandedly broke the Enigma coding machine that the Germans used to send coded messages. Of course this wasn't going to beat Hitler all by itself, but in so doing, Turing probably saved more lives than can be fathomed. Yet, after the war he was condemned for being unashamedly homosexual. Threatened with chemical castration, he took his own life. He also invented the computer, more or less. How this changed me was to further cement the idea that societally/religiously received ethics are often pure evil. Because one man's happiness disgusted some, he was condemned. The very man that had basically saved the whole country was condemned by that country.
5) The Great Transformation - Karen Armstrong
This book can probably be credited with me not being a militant atheist, while at the same time cementing a lot of disdain I have for the way a lot of religion is practiced and invoked.
4) The Blank Slate - Steven Pinker
Really shaped my thinking on the human mind, and how it works. Which is kind of a big deal.
3) Complexity - Mitchell Waldrop
To me, this book is about what makes things interesting. Things that don't change much are too boring, things that change too rapidly are just static. Things that ride the thin line of complexity is where all the magic happens. Profoundly shaped my views on intelligence, humanity, aesthetics and just about anything else you can think of.
2) Freedom Evolves - Daniel Dennett
This book was almost like a clarification of things I already thought. It really focused what I feel about free will and what we are.
1) The Christian Bible - Various (or God, depending on your views)
Living in a Western culture, even when The Bible isn't directly influencing your thought, you're usually thinking in reaction to it. For any person brought up in a Western Culture, to say that The Bible isn't their single largest influence is probably folly. You cannot help but be influenced by it. Regardless of if you are an Atheist, the most devout of Christians, Jewish or an agnostic.