Hard Sudoku puzzle books vol. 1: Hard, Very Hard and Extremely Hard Sudoku - Total 300 Sudoku puzzles to solve - Includes solutions (Hard to extreme)

Hard Sudoku puzzle books vol. 1: Hard, Very Hard and Extremely Hard Sudoku - Total 300 Sudoku puzzles to solve - Includes solutions (Hard to extreme)

Genius-Level Sudoku: Over 300 Super-Difficult Puzzles from the Japanese Masters Who Invented the Game

Brain Games - Extreme Sudoku