Book excerpt: From the original publisher of daily Sudokus in the United States, USA TODAY presents a collection of Sudoku puzzles in four difficulty levels, along with fun Sudoku variants, such as Sudoku X and Sudoku mini puzzles. USA TODAY Sudoku and Variants is a collection featuring Sudoku puzzles in four difficulty levels from USA TODAY--the nation's no. 1 newspaper, plus Sudoku X and Sudoku mini puzzles to round out the collection. Features include: Introductory how-to section 200 total Sudoku puzzles Comprehensive answer key