Sandwich Sudoku [SAND-4-004P]
Similar Puzzle Books
1000 Math Logic Puzzles – Collection II
US, CA; UK, DE, FR, ES, IT, other 
Puzzles in this book: Calcudoku, Sudoku XV, Vudoku, Mathrax, Sandwich Sudoku, Killer Sudoku, Terra X, Little Unique Killer Sudoku, Frame Sudoku, and Sum Skyscrapers.
