Sit & Solve® Double Black Diamond Hard Sudoku (Sit & Solve® Series)

Warning: danger ahead! These treacherously tough puzzles all require upper-level solving techniques to conquer. So, if you want hard sudoku that's easy to take on the go, this Sit & Solve® collection is the ticket. Attractively packaged, it's a terrific gift or stocking stuffer for hardcore sudoku fans.