Also, I really like the element where each shaded region must contain 1-6.
There's a 'wacky' version which is similar to 'jigsaw sudoku' and is also known as 'squiggly sudoku'.
Best of all: each version has difficulty levels ranging from very easy to very hard.
There are 500 levels, but there is no system of checking off which ones you've done.
I've bought 3 of the puzzle books at Peter's website. I've shared some of the puzzles with my math students.
Nice job Peter!
Update 10/7/2019: Peter informed me that there is an update to the game that includes level progress.