Simon & Schuster Mega Crossword Puzzle Book Series: This series offers a collection of challenging crossword puzzles with a wide range of difficulty levels. They are known for their variety and large format.
Will Shortz Presents... series: Will Shortz is a renowned crossword editor for The New York Times. He has published several crossword puzzle books with various themes and difficulty levels, including the "Will Shortz Presents KenKen" series and "Will Shortz Presents Sudoku."