Sudoku Unbound #1 ($2.99), is a new Kindle Active Content App from Puzzazz, the makers of
Wordoku Unbound #1 ($2.99), which I have (and like) and has all 5-star reviews. Since this should be essentially the same interface, but with traditional numbers instead of words, I think I'd ignore the lone 1-star review posted (complete with a nonsense title and misspellings in the body and was not posted by a verified purchaser).
Book Description
Crazy about Sudoku? Then this book of one hundred, fun, elegant, carefully crafted Sudoku puzzles is what you're looking for - these are some of the best Sudoku puzzles you'll find anywhere.
This Unbound puzzle collection gives you a great solving experience right on the page. Of course, you can use in-cell notes as you solve, just like you do on paper. If you're having trouble, move on to a different puzzle - you can always turn back to that page later. Or, get a hint and keep going.
You'll be both challenged and delighted with this book of puzzles that fit your personal skill level, because you can change from easy to medium to hard or back at any time. Whatever your skill and solving style, you'll appreciate that each puzzle can be played at three levels of difficulty.