I'm a big believer in the power of books. The most important books alter your life, or least your understanding of how the world works. I've read a lot of such books, but mostly a long time ago. The only one I've read this year was Jonathan Haidt's The Righteous Mind. I've written a good deal about that here, but if you want to understand the roots of moral behavior and the reasons for moral differences, I can't think of a better place to start. Take that, Plato, Aristotle, and Augustine.
The last book I can remember of similar impact was Jared Diamond's Guns, Germs and Steel. I'm a big fan of all his books, but this one for me is the go to place for understanding the underlying dynamics of history.
As a young person, I read Will Durant's The Story of Philosophy, which led to an infatuation with the subject, mercifully short lived enough that the only serious damage I accumulated was to my book buying budget. Robert L Heilbroner's The Worldly Philosophers, about economics and it's practitioners, did more lasting damage. Nowadays, though, I read mostly economics blogs and an occasional paper. It won't surprise any regular readers to hear that Paul Krugman's columns, blog, and books are at the core of what I think I understand about the subject.
I have always liked history, and three books that made early impressions were Momsen's History of Rome, Gibbon's Decline and Fall of the Roman Empire, and Thayer's The Influence of Seapower on History. I read less history these days, but it hasn't entirely lost its charm for me either.
Of course this is a tiny sample. Anybody else want to volunteer influential books?