You can order Escher books (and so much more) here. Many of the best books about Escher are out of print, so your best bet is to buy a used book through Amazon or AbeBooks.com.
This website will help you select a qualified conservator who can provide sound, ethical preservation services for your art objects, artifacts, and other items of historic and cultural value. The conservation professional can diagnose present and potential problems, provide treatment when necessary, and advise on appropriate conditions for storage and exhibition.
An indispensable, multi-dimensional website by Jill Britton, Tessellation Educator Extraordinaire, with links, great Escher Educational stuff, and lots more besides "If you only have time to check out one Escher link, make it this one!"
MOMA: What is a Print?
This is a very nice interactive explanation of printmaking techniques from MOMA, New York's Museum of Modern Art
A fine site about tessellations, by geometrician artist Andrew Compton, with many good links of its own, including information about seemingly every artist in the world doing tessellations.
Rinus Roelofs makes remarkable sculptures and animations which relate to illusions and Escher.
A huge gallery of Optical Illusions & Visual Phenomena by Michael Bach.
Ken Landry, the King of Polyhedra, has his own remarkable site, with poly-photos and much more.
M.C. Escher Biography
This biography is condensed mostly from the biography written by Bruno Ernst for the book M.C. Escher - His Life and Complete Graphic Work, © 1981, with some material from original essays by M.C. Escher.
Every possible impossible object, with many resources for creating impossible designs, with animations, programs and more. Don't miss the articles on Escher, including wonderful essays about mathematics in his art.
World of Escher
Escher Products, and a cool Escher site.
A great link for many illusions, sent to me by Ms. Walter's class.
The best site on illusions is "Illusionworks."
Fun Visual Tricks & Optical Illusions
Danielle, a student in Mesa Valley School District in Colorado suggested this resource
The M. C. Escher Foundation & The M. C Escher Company
The website of the M. C. Escher Foundation and The M. C. Escher Company: the copyright and trademark holder for M. C. Escher's art.
Derek de Solla Price is my late father. He was The Avalon Professor of the History of Science at Yale University from 1960 until his death in 1983. His writings on the history of science are fundamental to understanding the growth of science in culture. He was the first to record a complete lecture series on History of Science which could serve as a foundation for studies around the world before the internet. His entire 1976 lecture series "Neolithic to Now" is now available on-line and for free at derekdesollaprice.org