The June/July 2010 issue of the AMS Notices here has a further article by Doris Schattschneider (a graduate school classmate of mine) on Coxeter and Escher. Doris has written extensively about Escher's work from a mathematical viewpoint, so it's worth checking out her point of view.
P.S. Don't overlook in this Notices issue Bill Casselman's column About the Cover.

