Below are sources that you might find useful if you want to learn more about the Mathematics of Sudoku.

•http://www.afjarvis.staff.shef.ac.uk/sudoku/sudoku.pdf
An excellent article on Sudoku counting by Bertram Felgenhauer and Frazer Jarvis, understandable for anyone who puts his mind to it.

•Wikipedia: The Mathematics of Sudoku .
Available from http://en.wikipedia.org/wiki/Mathematics_of_Sudoku

•F. Crook: A Pencil-And-Paper Algorithm for Solving Sudoku Puzzles.
Crook’s 2009 paper describing his algorithm for solving Sudoku puzzles. Available from http://www.ams.org/notices/200904/rtx090400460p.pdf .

•www.veryfreesudoku.com
A website with thousands of free Sudoku puzzles of all degrees of difficulty.

•Agnes M. Herzberg and M. Ram Murty: Sudoku squares and chromatic polynomials.
A 2007 paper that appeared in the Notices of the American Mathematical Society, volume 54, issue 6, pages 708-717. This paper isn’t light bed-time reading...