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15 Minute Speed Sudoku - 175 hard sudoku puzzles.: sudoku,puzzle,hard,difficult,gift

Only Hard Sudoku: 400 Challenging Puzzles

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25x25 Sudoku: 100 sudoku puzzles complete with solutions

The Must Have 2018 Sudoku Puzzle Book: 2018 sudoku puzzle book for 365 daily sudoku games. Sudoku puzzles for every day of the year. 365 Sudoku Games - 5 levels of difficulty (easy to hard)