The Mathematical Structure of Escher’s Print Gallery (2003) [pdf]
ams.org
ams.org
https://web.archive.org/web/20130422193857/http://escherdros...
https://www.ams.org/notices/200304/fea-escher.pdf
It explains all the math, shows Escher's work drawings, etc. Escher himself didn't seem to be completely cognizant of the math, so he worked by intuition and eyeballed things, but he came to a very close approximation of what the mathematicians worked out decades later.
Fun fact: I live within 500 meters of where Escher was born :), nice to see him pop up here on HN. Highly recommended to go see his other works if you haven't yet.