Combinatorics Journal Two articles: R17, "Escher's Combinatorial Patterns" by Doris Schattschneider R23, "On a Tiling Scheme from M.C. Escher" by Dan Davis.
Steve Ogden's implementation of Ellen Gethner's colouring algorithmFor Ellen's PhD she devised an algorithm for colouring patterns based on these tiles. Steve has implemented this in Java so that you can play it with on your computer. You must check this out, it's great!
Rui Menino's fantastic Eschertile program for Windows!Rui has created a wonderful program for you Windows users out there. It allows you to create your own patterns and tile them in the same ways that Escher did. The site is in Portugese so here is the English google translation. I recommend this program to everybody who is interested in exploring this tiling method.