The New York Public Library is digitizing its collection of New York City Directories, 1786 through 1922/3, serving them free through the NYPL Digital Collections portal. The first group, 1849-1903, is already available. Directories have long been an important tool for business historians, recording the names and addresses of residents, of churches, businesses, schools, police stations, courts, and other government offices, as well as individual names associated with those institutions.
See the post by Philip Sutton on the NYPL blog for a longer discussion of the value of city directories for historians and tips for using them.
City directories contain much more than lists of names and addresses. They record the price of travel and postage, the kinds of occupations undertaken in the city, the layout of streets, and at what time the sun was predicted to rise and set. . . . In addition to textual information, city directories feature many images, including maps, illustrations of buildings, and advertisements, occasionally printed on colored or decorative paper. Directories record the city's built and commercial history.