1. Platonic Realms Mini Text
"The Worlds of M.C. Escher"
http://www.mathacademy.com/pr/minitext/escher/index.asp
The first page of this site has a clear description of the principles of Escher's art.

2. The National Gallery of Art
http://www.nga.gov/collection/gallery/ggescher/ggescher-main1.html
The gallery is in Washington, D.C. The web site has a comprehensive section about Escher with many very high quality images.

3. Cordon Art
http://www.mcescher.com/
This Netherlands art company is the copyright holders for Escher's works.

4. Worlds of Escher
http://www.worldofescher.com/
In addition to lots of images of Escher's works, if you ever want to buy anything related to Escher it is probably here.