[mrmyers]

While his puzzle books obviously deserve a lot of praise, Smullyan's textbooks and papers definitely shouldn't be overlooked. There's a lot of wonderful gems to be found there.

Diagonalization and Self-Reference is the single book I would recommend the most to the HN crowd. There are a few sections on quotation and Quines that I've found endlessly useful, his 'Elementary Formal Systems' is my favorite presentation of computability, and there's a lot of really deep stuff in there about the interaction between incompleteness, uncomputability, and fixed-points.

Also, Logical Labyrinths is a pretty great textbook on formal logic. The first half is in the form of one of his puzzle books, introducing notions and building intuitions, while the second half builds off of them to provide a more formal perspective, while incidentally giving a kind of eye opening look at how he comes up with his puzzles and how they map to certain deeper properties of logic.