If you like puzzles and are looking for something more challenging than the daily crossword, I highly recommend The Lady or the Tiger by Raymond Smullyan. The title references the famous short story of the same name, and features a collection of progressively challenging logic puzzles. The first half of the book should be engaging enough for any enthusiastic puzzle lover. Only serious mathmaticians should apply themselves to the second. Sadly the book is out of print, but you can get a used copy easily.