Douglas Hofstadter has spent a lifetime thinking about the way we think. He's written about it perhaps more than anyone else in the past century. Even if someone out there has beaten him in sheer quantity of words on the subject, nobody has come close to rivaling his style or his impact on programmers everywhere.
All of his books are wonderfully imaginative and are loads of fun to read, but if you're a programmer and you haven't yet read Gödel, Escher, Bach: An Eternal Golden Braid (usually known as "GEB"), then I envy you: you're in for a real treat. Get yourself a copy and settle in for one of the most interesting, maddening, awe-inspiring and just plain fun books ever written. The Pulitzer Prize it won doesn't nearly do it justice. It's one of the greatest and most unique works of imagination of all time.