It's always good to see something from the folks at Leiden. It's also nice to see that M.C. Escher was hesitant to recurse at such small length scales, and to see clever folks use computers and holomorphic functions to solve the problems which he himself found puzzling.
I know it's not technically mathematical, but someone in this community should point to Godel, Escher, Bach by Douglas Hofstadter. It is a fascinating book that partially explores the complexities within Escher's work.