Places are largely defined by the people who live and work there. You can learn a lot about the character of a place and how a place has changed over time by exploring demographic trends (statistical information about human populations) and notable community members like politicians, business leaders, or landowners.
Historic Census Data: Digitized raw census data from 1930 and 1940; and information about census collecting dating back to 1790. Because of privacy restrictions, there is a 72 embargo on raw census data.
American Factfinder: Population, Demography, Economic, & Housing Data searchable by location. Factfinder data derived from U.S. Census, Government Population Estimates, and American Community Survey (ACS), American Housing Survey (AHS).
Social Explorer (Library Login Required): Database with mapping tools to visualize demographic information (race, occupation, income, education, etc) in the United States.
Ancestry.com (available on site at all NYPL libraries): Searchable census and vital records (birth, marriage, death). Also includes: the Social Security Death Index, passenger lists and naturalizations, military and Holocaust Records, city directories, New York Emigrant Savings Bank records, and African American and Native American records.
Vital Records (birth, marriage, death): Original vital records are available in personal at the NYC Municipal Archives.
Genealogy Research: The National Archives and Records Administration (NARA) retains immigration records, passenger lists, passport records, land records, military records. See these FAQs more information about using NARA collections for geneaology research.
With a few exceptions, very few local politicians and individuals have whole books written about them. Tertiary sources (like encyclopedias and directories) and newspapers are usually the best place to begin conducting this type of research. Original documents produced by or about notable people and politicians are often collected by archives.
Encyclopedias (require library login)
Manuscript Collections in New York City