When I’m puzzling out the ID of a hairstreak or swallowtail, I depend on Cech and Tudor’s essential Butterflies of the East Coast: An Observer’s Guide (2005). The book has one significant flaw, however: to look up a species, the index doesn’t indicate the page where the main entry appears. No boldface or italics. The index for Monarch cites 24 different pages.
So, I fixed it, in part: Index to Cech and Tudor, Butterflies of the East Coast: An Observer’s Guide, main entry, by common name.