This book contains 100 Double CalcuDoku puzzles.

CalcuDokus, introduced in 2004 as KenKen¨ (a registered trademark of Nextoy LLC), is a 9x9 numeric puzzle similar to Sudoku. But, unlike Sudoku, CalcuDoku doesn't require you to learn complex strategies. A Double CalcuDoku is obtained by overlapping two CalcuDoku puzzles.

It's a great puzzle that is fun to play, and there are no other books that let you play it.