The Times Super Fiendish Su Doku Book 4: 200 of the Most Treacherous Su Doku PuzzlesPaperback, 1 August 2017

The 200 puzzles in this collection of treacherously difficult puzzles will stretch even the most advanced Su Doku enthusiast. You will need to use all of your best solving techniques to get to the end of this testing challenge. The puzzles in the collection are of the highest quality and are perfect for the advanced solver in need of a constant supply of ultra-difficult puzzles.