William S. McFeely, a historian who won a Pulitzer Prize for his biography of Ulysses S. Grant but was also well known for advancing the field of black history, died on Wednesday in Sleepy Hollow, N.Y. He was 89.

His son, W. Drake McFeely, said the cause was idiopathic pulmonary fibrosis, a lung disease.

Professor McFeely also wrote an acclaimed biography of Frederick Douglass as well as “Yankee Stepfather: General O.O. Howard and the Freedmen” (1968), a study of the Freedmen’s Bureau, set up by the government at the end of the Civil War to oversee the welfare of freed slaves, and the man who ran it.

These books and other writings established Professor McFeely as a leading interpreter of Reconstruction, the pivotal period after the Civil War.

“Via his books on Howard, Douglass and Grant,” the historian Eric Foner said by email, “McFeely played a major role in the re-evaluation of Reconstruction — seeing it not as an era of misgovernment and corruption as previous scholars too often did, but as a key moment, despite its flaws, in the ongoing struggle for racial justice in this country.”