Both the 5x5 and 6x6 puzzles of the "Difficult" level on the Calcudoku website are popular because they are tricky: small but challenging, such that even experienced puzzlers need at least 10-15 minutes to solve one. This book has 100 of each, plus one tricky 7x7 puzzle (so 201 in total). The 6x6 puzzles are all in a regular pattern (there are 10 different patterns). The puzzles are ordered by level of difficulty. If you're not familiar with the rules of Calcudoku, then go to www.calcudoku.org and try some puzzles there. At the site, you can sign up for an account, and score points for solved puzzles. You can also submit solutions for the last 41 puzzles of this book on the site, and score 5 points for each correct solution.