Number Galore for Adults to Explore 240 Sudoku Puzzle Books Hard EditionPaperback, 20 September 2017

Hard Sudoku games require more time thinking and planning. Come up with a good strategy to follow and stick to it. To come up with a strategy, you will have to do a lot of brain conditioning through trial and error. Are you up for it? If you are, then here's the book to begin with!