References:
1. Zachary Pitkow, Sudoku: Medium to Hard, Chronicle Books, 2006.
2. Frank Longo, Absolutely Nasty Sudoku Level 4 (Mensa), Puzzlewright, 2007.
3. Peter Gordon, Frank Longo, Mensa Guide to Solving Sudoku: Hundreds of Puzzles Plus
Techniques to Help You Crack Them All, Sterling, 2006.