The Little Book of Sudoku Volume 5: Hard-As-Nails

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 skills, improve their memory and keep their brain healthy on your daily commute, at the airport, on vacation, or in the comfort of home.