Butterfly Websites Guides
KAUFMAN Field Guide to Butterflies of North America by Jim P. Brock & Kenn Kaufman
Peterson Field Guides Western Butterflies by Paul A. Opler/ Amy Bartlett Wright
National Audubon Society Field Guide to North American Butterflies by Robert Michael Pyle, Alfred P. Knopf
I would recommend these field guides in this order. However, an enthused butterfly gardener will want all three. They all provide great information in different formats, so depending what you are looking for a particular guide may be better.
These are the only guides I have purchased and used so I cannot comment on any others out there.
TAMU Insect Field Guide Go Aggies!