He is the author of seven butterfly field guides that use photographs of live butterflies, in addition to initiating and editing a field guide to dragonflies and one to wildflowers.
In 1999, Robert Robbins, then chair of the Department of Entomology at the Smithsonian Institution's National Museum of Natural History, stated that "Glassberg's butterfly guides have provided a new way for amateur naturalists to identify species without collecting them or dissecting them.