Absolutely Nasty® Sudoku Level 4

Frank Longo creates, fact-checks, and edits crossword puzzles, and also specializes in sudoku, having published more than 150 sudoku books. His books have sold more than 5 million copies. His wide array of titles encompasses both standard and variety sudoku puzzles, and includes Beyond Black Belt Sudoku, Way Beyond Black Belt Sudoku, The Nastiest Sudoku Book Ever, Pause for Puzzles: Sudoku for Mindfulness, Sip & Solve Simple Sudoku, Blockbuster Book of Sudoku, Puzzlewright Guide to Solving Sudoku (with Peter Gordon), and Skill Builder Sudoku (with Peter Gordon).