The Library of Congress’ collection of telephone directories represents the following states and localities: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, the District of Columbia, Florida, Georgia, Hawaii, Iowa, Maryland, Pennsylvania, and the city of Chicago. The dates of the directories span most of the 20th century. The Library’s United States telephone directory collection consists of 8,327 digitized reels of microfilm; of these, about 3,500 are presented in this collection. The remainder of the collection may be requested from the Microform Reader Services (LJ 139).
You can read more about this online collection at https://www.loc.gov/collections/united-states-telephone-directory-collection/about-this-collection/ while the catalog of available directories and much more may be found at https://www.loc.gov/collections/united-states-telephone-directory-collection/.
My thanks to newsletter reader Michelle Cadoree Bradley for telling me about this online resource.