The Struggle for Equality: Abolitionists and the Negro in the Civil War and Reconstruction. Princeton, NJ: Princeton University Press, 1964.
The Negro's Civil War: How American Negroes Felt and Acted During the War for the Union. New York: Pantheon Books, 1965.
Marching Toward Freedom: The Negro in the Civil War, 1861-1865. New York: Knopf, 1968.
The Abolitionist Legacy: From Reconstruction to the NAACP. Princeton, NJ: Princeton University Press, 1975 (1st ed.); 1995 (2nd ed., with a new preface by the author).
Ordeal by Fire: The Civil War and Reconstruction. New York: Knopf, 1982 (1st ed.); New York: McGraw-Hill, c1992 (2nd ed.); c2001 (3rd ed.), c2009 (4th ed.).
Lincoln and the Strategy of Unconditional Surrender. Gettysburg, PA: Gettysburg College, 1984.
How Lincoln Won the War with Metaphors. Fort Wayne, IN: Louis A. Warren Lincoln Library and Museum, 1985.