Hard Sudoku Puzzle Book for Adults This sudoku book is a perfect tool to train memory and logic and increase concentration. It's the perfect gift for all sudoku fans. The interest of the game lies in the simplicity of its rules, but in the complexity of its solutions. All puzzles have a unique logical solution. All answers are included at the end of the book. The level of problems has been carefully Level Hard to extreme assessed 4 problems per page Total 300 sudoku puzzles: 50 hard sudoku 50 Difficult sudoku 100 very hard sudoku ( Insane ) 100 extremely hard sudoku ( Inhuman )