This book has 250 of the popular Killer Sudoku puzzles, ranging from easy to very challenging. There are five categories: "Warmup" (with 25 puzzles), "Easy" (50), "Medium" (50), "Tricky" (50), and "Difficult" (75) (so no puzzle is actually impossible: -). Solving a Killer Sudoku takes a combination of Sudoku-style logic reasoning and basic math skills, making it a much more interesting puzzle. The puzzles are by the author of the popular Calcudoku puzzle books. Solve both Calcudoku and Killer Sudoku puzzles online at calcudoku.org and score points! You can even submit solutions for the difficult puzzles in this book, and score points for each correct solution. The 1st edition of this book has the same puzzles, but without the solutions in the back. This 2nd edition does have all solutions in the back of the book.