Absolutely Nasty® Sudoku Level 4

Frank Longocreates, 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 includesBeyond 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).