As many of you may know, Sudoku is a logic-based, combinatorial number-placement puzzle. I recently found an online database of Sudoku puzzles at http://www.websudoku.com/.
Sudoku has a great deal of math involved in it. If you are interested in reading about the mathematics behind Sudoku, you might want to check out Taking Sudoku Seriously: The Math Behind the World’s Most Popular Pencil Puzzle by Jason Rosenhouse and Laura Taalman. I have not yet read the book myself, but have heard great reviews on it. Let me know what you think about it if you have read it.