Whether you call them hard, difficult, fiendish, devilish, migraine or black belt, these are puzzles for people who need more of a challenge.
Times Killer Sudoku 1-9
Nine volumes of previously unpublished puzzles for hard-core Su Doku addicts. The puzzles use the same 9x9 grid but there is a double challenge - it is also necessary to ensure that the outlined cubes add up to the same number.
Black Belt Sudoku By Michael Rios. Taking a lead from karate--another Japanese art--each title is graded by color: White Belt for easy, Green Belt for medium level, Brown Belt for hard, and here is Black Belt for the super-tough solvers. Contains 300 puzzles.
Sudoku Quotes and Tips The great thing about these puzzles is that they have to be solved by pure logic, not luck. They can be frustrating, but the reward when you crack them is a great feeling. Wayne Gould, Father of the current Sudoku craze.