Certainly R. Penrose and Emperor's New Mind! It's extremely amusing, in-depth (compared to most popular science books) and very broad (like GEB, it touches many topics in Mathematics, Computer Science and Physics).

Penrose not only synthesises what's known, but presents many of his own findings and I find myself coming to the book from time to time as a reference. Finally, the book quickly becomes philosophical, discussing philosophy of mathematics, consciousness and cosmology. Author clearly indicates which ideas are merely his philosophical thoughts, which allows reader to disagree and have fun thinking about his propositions.