Every puzzle has an unique solution. At any point, you can find a empty square where the correct number can be written. You do not need to guess.
My friend gave the 2 books to his friend who likes Sudoku. The friend completed the puzzles in the 2 volumes within a few days. He took 6 minutes to complete each puzzle. He wants more.
The puzzles are good for elders to exercise their mind. They are printed in large font, which is good for the eyes.
Price $8 (postage is free)