Identification, Images, & Information
For Insects, Spiders & Their Kin
For the United States & Canada
Butterflies of Pennsylvania, a field guide
By James L. Monroe, David M. Wright
University of Pittsburgh Press
ISBN: 9780822964551
Cite: 2117542 with citation markup [cite:2117542]
From the publishers page:

This work has all of the features that make field guides to a region's butterfly fauna useful to anyone with a serious interest in that fauna. . . . the book is a bargain and a must for anyone with an interest not just in Pennsylvania's fauna, but the northeast fauna as a whole.
News of the Lepidopterists' Society

Winner, 2017 National Outdoor Book Award


Though I am in the southeast, I own this field guide and find it very useful - Roy Cohutta Brown.