Web sites for researching ZIP code and explaining ZIP codes:
Zip Codes are Not Area Polygons
article from the maker of
Manifold
GIS.
ZIP Code Boundaries
from
Maponics
, a provider of mapping services and GIS data for businesses.
ZIP-CODES.com
is a helpful site for ZIP code research including
Tucson area ZIP codes
ZIP code research
from
city-data.com
.