1001 Hard Sudoku Puzzles

Will Shortz Presents The Huge Book of Hard Sudoku: 300 Challenging Puzzles

Only Hard Sudoku: 400 Challenging Puzzles

Beyond Black Belt Sudoku: If you have to ask, it's too hard for you. (Martial Arts Puzzles Series)

Sudoku 3: 200 Puzzles: Hard to Extreme

Will Shortz Presents Surrender to Sudoku: 200 Irresistibly Hard Puzzles

Hard Sudoku Book: 200 Difficult Sudoku Puzzles (Volume 1)

The Must Have 2016 Sudoku Puzzle Book: 366 puzzle daily sudoku book for the leap year. A challenge for every day of the year. 366 Sudoku Games - 5 levels of difficulty (easy to hard)

SUDOKU VERY HARD PUZZLES Volume 2: 400 Sudoku Puzzles - Very Hard Level (Sudoku Hard Puzzles)