The difficulty level soars, as even more tricks get added to the mix: X-Wing; Swordfish; XY-Wing; XYZ-Wing; and the Gordonian Rectangle.

The vast majority of sudoku puzzles that are labeled "hard," "expert," "challenger," are not really all that difficult. And in a surprising number of cases, they are downright easy. Part of the "Absolutely Nasty Sudoku" series, this book features basic and intermediate puzzles and techniques.

Watch outthese sudoku might crush even the most stalwart of solvers

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