It's hard to name just one, but off the top of my head, a few obvious possibilities include:<p><i>The Selfish Gene</i> (Dawkins) - helped nudge me over the line in terms of really identifying as an atheist, by satisfactorily refuting any lingering hints of the "irreducible complexity" argument that were lodged in my brain.<p><i>The Fountainhead</i> (Rand) - I actually saw the movie before reading the book, but the Howard Roark character is a real role model character to me.<p><i>The Four Steps to the Epiphany</i> (Blank) - As far as I'm concerned, this is <i>THE</i> book for startup founders / entrepreneurs. Very detailed, very logical, iterative process for building a company, defining a product, finding customer base, etc.<p><i>Nineteen Eighty-four</i> (Orwell) - Read this one my senior year in High School and it left a profound mark on my psyche. I was already leaning towards a very individualistic / libertarian worldview, but 1984 really enflamed my hatred for big, heavy-handed, oppressive government(s).