There are a large number of institutions in Philadelphia that maintain historical records relating to the city and region. For a list of the major institutions, with brief descriptions of their holdings and links to their websites, see the site for the Philadelphia Area Consortium for Special Collections Libraries.
The most important institutions for Philadelphia topics are:
Contains address information about individuals and businesses, sometimes organized by street address, and sometimes alphabetically by name.
American city directories published before 1820 are available online through Early American Imprints.
A full set of 19th and early 20th century Philadelphia directories can be found in the Quaker Collections at Haverford College, as well as at the major historical libraries in the city.
Bryn Mawr also has microfiche copies of directories from Philadelphia, New York, and several other cities up to the mid-19th century (kept in microfiche cabinet in Periodicals Room, 1st floor of Canaday.)
Detailed atlases were produced for most American cities and counties in the late 19th and early 20th centuries. The Bryn Mawr and Haverford libraries have many of the published atlases, and others can be found at the Free Library, Library Company, and Historical Society of Pennsylvania. Several companies also produced fire insurance maps, which show in detail the outlines of the buildings, the construction materials, use of the buildings, and public infrastructure. A few of these maps are online (below), and others can be found at the Free Library and the Historical Society of Pennsylvania.