Most of the "eye-opening or incredibly interesting" philosophical material that I've read has been in individual articles rather than books. That said, here are a few favourites from various areas. It's a bit heavily weighted towards recent material; Kant and Plato ought to be on there, but I can't stand Kant as a writer, even if his ideas are profound, and Plato's style has always irritated me for some reason I can't quite put my finger on. I could probably keep tying titles all day, so I'll stop with those that first popped into my head. Aristotle, Nicomachean Ethics David Chalmers, The Conscious Mind: In Search of a Fundamental Theory Gottlob Frege, Die Grundlagen der Arithmetik Thomas Hobbes, Leviathan David Hume, An Enquiry Concerning Human Understanding J.S. Mill, On Liberty John Rawls, A Theory of Justice Stewart Shapiro, Philosophy of Mathematics: Structure and Ontology
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