i've used a lot of disparate internet sources from googling and duckduckgo-ing like wikipedia and whatnot, but i've also got a few sources i use consistently for reference! here they are. hooray for the internet and the library!
Go Botany
HOSTS - a Database of the World's Lepidopteran Hostplants
UMass Amherst's Weed Herbarium
iNaturalist
Wildlowers in the Field and Forest (Clements & Gracie; 2006)
A Guide to Invasive Plants in Massachusetts (Somers, Kramer, Lombard, Brumback; 2008)
Weeds of the Northeast (Uva, Neal, & DiTomaso; 1997)
Wildflowers of New England (Elliman & New England Wild Flower Society, 2016)
The Audubon Society Field Guide to North American Wildflowers, Eastern Region (Knopf, 1985)
Wild Urban Plants of the Northeast (Del Tredici, 2010)
Our Northern Shrubs and how to identify them (Feeder; 1969 republication of 1903 edition)
Field Guide to Wildflowers: Northeastern and North-Central North America: Peterson Field Guide (Peterson/McKenny, 1998)
i'm always on the lookout for plant ID guides at thrift stores and yard sales and free piles and such.
the photos of plants were all taken by me with my phone, and this site's little tree favicon is from mazeguy smilies.