X-Treme Sudoku
Original Sudoku
More Original Sudoku (Bk. 3)
The Original Sudoku Book 2 (Bk. 2)
Beyond Black Belt Sudoku: If you have to ask, it's too hard for you. (Martial Arts Puzzles Series)
Extreme Sudoku: A collection of 200 of the toughest Sudoku puzzles known to man. (With their solutions.)
SUDOKU DIABOLICAL Volume 1: Diabolical Sudoku: 400 sudoku diabolical puzzles, difficult level for extreme sudoku puzzle enthusiasts (Sudoku evil, very hard)
Will Shortz Presents Genius Sudoku: 200 Extreme Puzzles
Will Shortz Presents Surrender to Sudoku: 200 Irresistibly Hard Puzzles