My favorite Smullyan book is out of print: Forever Undecided: A Puzzle Guide to Gödel. Gödel's paradoxical theory hooked me in college, and after twenty years I

My favorite Smullyan book is out of print: Forever Undecided: A Puzzle Guide to Gödel. Gödel's paradoxical theory hooked me in college, and after twenty years I still can't help but feel that it has a message for us---not in the word-play associations that some philosophers mistreat such things, but more in its implications for the human mind and artificial intelligence, along the lines that Roger Penrose explores. I've read Franzén's rejection of all attempts to "apply" Gödel's proof, and yet, I keep coming back to the idea that we can see our way "out" of these limited systems. Anyway, Smullyan's book is a real treat. In the later chapters he even goes beyond Gödel. I've never completely figured out how much of that is Smullyan and how much is other published research.