HardAsNails Sudoku Volume 5 Author:Gareth Moore More than 200 advanced puzzles are broken down into five distinct levels of difficulty, with each succeeding level requiring a deeper understanding of the puzzle to make progress. All are guaranteed to rack the brains of even the most practiced Sudoku addict. The pocket-sized format means that solvers can test and build their logic and reasoning... more » skills, improve their memory and keep their brain healthy on your daily commute, at the airport, on vacation, or in the comfort of home.« less