If Classic Sudoku can not satisfy you anymore, try the X-Sudoku Level Expert.You have new challenge for diagonal!In Expert level, you have 21-23 known numbers.Good luck! X-Sudoku Expert level vol.1 Details - Small Print (5x8) to easy take with - 100 Expert Level Sudoku Puzzles Collection- You can challenge yourself by writing down the time, you used. -Each puzzle have 1,2,3,4,5,6,7,8,9 on the top.It is for you when you clear one number, Bang it offThis series have four different levels: easy, normal, hard, and expert. This book is expert level. if you need other level, please check "Mike Murphy Sudoku". Enjoy!