30 books • 6 series
Medium Sudoku Puzzles Volume 3 (Medium Sudoku Puzzles, #3)
Hard Sudoku Puzzle Book Volume 2
Hard Sudoku Puzzle Book Volume 1 (Hard Sudoku Puzzle Book, #1)
Hard Sudoku Puzzles Volume 3
Hard Sudoku Puzzles Volume 1
Hard Sudoku Puzzles Volume 2