The Concept of Mind. If any one book killed cartesian dualism, this was the one book that killed cartesian dualism.

Laszlo Mero, Moral Calculations. Both for why you might want to do ethics with game theory, and as an example of why you want to resist the temptation.

Robin Hartshorne, Geometry: Euclid and Beyond. Coxeter made geometry a respectable subject again, but this is the modern geometry book.

Thomas Ligotti, Songs of a Dead Dreamer. This is the book I loan people who think all horror is schlock.

William Goldbloom Bloch, The Unimaginable Mathematics of Borges' Library of Babel (and, implicitly, the Library of Babel itself). No reason other than I enjoyed it.

E.T. Jaynes, Probability: The Logic of Science. Ignoring that if not for Jaynes we probably wouldn't have Less Wrong, and the world would be a better place, this is both an excellent explanation of the logical interpretation of probability by the master and why many newly minted Bayesians (and not-so-newly-minted charlatans) act like they just got religion.