Collection of Puzzles
For those who are looking for Puzzle books, forget it. William Wu's site is one of the best puzzle sites on the net. He has made a comprehensive archive of riddles that are very challenging and loads of fun. Also useful for interview preparation. PC, you won't feel the need for a puzzle book after browsing his puzzles page.
PS: Do you know any good site with challenging puzzles/riddles? Do use to comments section to let me know.