Guide to American Medical Biography

City directories can be used to obtain a list of all physicians for a community or to verify the presence of a particular person.

Characteristics: American city directories

• have been published since the late 18th century. 
• generally have a classified terminal section with headings for physicians, midwives, etc.  
• Doctresses (female physicians) were sometimes not listed in the classified occupation section.  
• Pre-1960, people of color were often listed in separate sections or had asterisks next to their names. 

This book offers a discussion on the biases and limitations of city directories:
The plain people of Boston, 1830-1860: a study in city growth (print only), by Peter R. Knights. New York: Oxford University Press, 1971, 204 p.

In addition to the pre-1923 works in the free digital libraries, there are subscription genealogical databases and extensive microfilm collections. More information in Direct Me NYC 1786: A History of City Directories in the United States and New York City, by Philip Sutton (NYPL).

The development and growth of city directories, by A. V. Williams. Cincinnati, Ohio: Williams directory co., 1913, 152 p. | WorldCat Record

Ancestry.com (Ancestry Library Edition) includes many city directories: choose Schools, Directories & Church Histories, then City & Area Directories. Then refine by geography and/or date. There are UK as well as American directories.

United States Online Historical Directories collects links to online editions of city directories.

HathiTrust. Search for directories in the Catalog with the name of you city and the term directories.  Search within a directory with Advanced Full Text Search.

City directories of the United States. 4,586 microfilm reels. Woodbridge, CT: Research Publications; Primary Source Media, 1970- .
Segment 1. American directories through 1860 (6820 microfiches) -- segment 2. 1861-1881 (389 reels) -- segment 3. 1882-1901 (771 reels) -- segment 4. 1902-1935 (2008 reels) -- segment 5. 1936-1960 (325 reels).

Guide: Bibliography of American directories through 1860 (print only), by Dorothea N. Spear. Worcester, Mass., American Antiquarian Society, 1961, 389 p. 

U.S. City Directories on Microfilm in the [Library of Congress] Microform Reading Room (bottom of page) lists cities with available directories