Kelly Johnson 
Sudoku A Game of Mathematicians 400 Puzzles Normal and Hard Difficulty [EPUB ebook] 

Enjoy the Sudoku game with 400 normal and hard puzzles. Hours of entertainment. Makes a great gift. An amazing way to develop and practice problem solving skills. Change the boring moments of waiting for someone or the day that is stuck in the house, can’t go out because of heavy snow. This book will help kill those boring time into a relaxation and keeping your memory sharp.