The author of "To Mock a Mockingbird", Raymond Smullyan, has written extensively on logic, and he has books all across the spectrum of difficulty. He even wrote

The author of "To Mock a Mockingbird", Raymond Smullyan, has written extensively on logic, and he has books all across the spectrum of difficulty. He even wrote what some consider to be "The Hardest Logic Puzzle Ever". http://en.wikipedia.org/wiki/The_Hardest_Logic_Puzzle_Ever http://en.wikipedia.org/wiki/The_Hardest_Logic_Puzzle_Ever