In addition to federal census records, state census records can also be incredibly helpful. This is especially true for the African American population in the United States, considering the mass migration that took place during and after Reconstruction. State census records might also include more nuanced questions and information that was not included in federal census records.
In addition to the state census records, there are many supplemental books and indexes that are accessible from the Schomburg Center and other library organizations. Below are some examples:
Slaves -- New York (State) -- Genealogy.
Ohio -- Census, 1880.
New York -- Genealogy.
African Americans -- South Carolina -- Genealogy.