Skill Builder Sudoku: Hard to Expert

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 Pause for Puzzles: Sudoku for Midfulness, Sip & Solve Simple Sudoku, The Peaceful Mind Book of Sudoku, Blockbuster Book of Sudoku, Beyond Black Belt Sudoku, and Puzzlewright Guide to Solving Sudoku (with Peter Gordon).

Peter Gordon has written and edited more than 50 puzzle books, with combined sales of over a million copies. From 2002 to 2008, Gordon was the crossword editor for the New York Sun. He has written more than 120 New York Times crosswords.