Hard Sudoku Book: 200 Difficult Sudoku Puzzles (Volume 1)
Only Hard Sudoku: 400 Challenging Puzzles
Will Shortz Presents The Huge Book of Hard Sudoku: 300 Challenging Puzzles
Sudoku Genius: 144 of the Most Fiendish Puzzles Ever Devised
Sudoku 3: 200 Puzzles: Hard to Extreme