As Phil mentioned, Bookfinder is a great resource for finding reading
and reference material.  I would like to add another search option
called http://www.abebooks.com/.  Abebooks runs the same way as
bookfinder.  Many antiquarian and used bookstores choose either
Abebooks or Bookfinder to host their books, so it's important to check
both websites to find the cheapest/available seller.

Jim Brighton
Easton, MD