X-Treme Sudoku by Nikoli Publishing

X-Treme Sudoku : Paperback : Workman Publishing : 9780761146223 : 0761146229 : 27 Jul 2007 : The Sudoku craze continues! This work contains the hardest 300 puzzles, which are written by the people who introduced the puzzle to Japan in the 1980s - and who hold the trademark for Sudoku in Japan.