Incorporated in 1905, the National Audubon Society is a nonprofit conservation organization that uses education and advocacy to advance its mission to conserve and restore natural habitats of birds and wildlife in the United States and across the Americas. Audubon also produces bestselling descriptive field guides on a wide variety of nature-related topics.
I read field guides for fun. I throughly enjoyed this one. I hope a revised edition come out soon. This was originally written in 1981 and a lot has changed since then, especially the plight of the Monarchs.
The Audubon Society Field Guide to North American Butterflies by Robert Michael Pyle (Knopf 1991)(595.789). Here is a field guide with photographs to identify over 1000 species of butterflies. My rating: 7/10, finished 2005.