Amazon has Sudoku Volume 1: Interactive Sudoku Puzzles for Kindle 2 and Kindle DX for FREE (regularly 99-cents) for a limited time.

Each Sudoku volume contains 20 different Sudoku puzzles: five puzzles for each difficulty level.

The objective of a Sudoku game is to fill a 9×9 grid so that the numbers 1 through 9 occur exactly once in each row, column, and 3×3 box.