This is a book with Level 6 Sudoku puzzles, the toughest ones. If you think they may be too tough for you, pick the book with a different book cover color - a different level (from 1 to 6). Find the level that fits you best, and enjoy your time well spent. Either you want - to exercise your brain to boost your focus and mental abilities, or - to prevent early memory loss and to lower the risk of Alzheimer's disease, and to keep your mind young for years to come, or - to relieve your stress and to relax after a hard day at work, or even - to prepare for the next World Sudoku Championship, or just - to kill a couple of hours on a plane - This book series is for you. HAPPY SOLVING!