About a year ago I stumbled across “irregular Sudoku” books in Brazilian newsstands. Of the many variations on the game, I enjoyed “Killer Sudoku” the most. In addition to the regular Sudoku rules (filling a 3x3 square with the numbers 1 to 9), this game contains few if any number clues and involves much more arithmetic. It’s HARD and a good alternative if you’ve become bored with regular Sudoku.
The book pictured is the only one I have found in the U.S. so far. It’s more difficult than the ones I am used to (I almost always have to look in the answer pages to make sure I’m on the right track), but it’s still a blast. When I finished it, I turned right around and ordered a second copy!