Absolutely Nasty Sudoku Level 2
Absolutely Nasty Sudoku Level 4
Absolutely Nasty Sudoku Level 1
Beyond Black Belt Sudoku: If you have to ask, it's too hard for you. (Martial Arts Puzzles Series)
mental_floss Sudoku: It's the Brain Candy You've Been Craving!
Sudoku 3: 200 Puzzles: Hard to Extreme
Puzzlewright Guide to Solving Sudoku: Hundreds of Puzzles Plus Techniques to Help You Crack Them All
Will Shortz Presents Surrender to Sudoku: 200 Irresistibly Hard Puzzles