Jesse J. Holland of the AP has written a fascinating story on the black spies who gathered information on the Confederate Army during the Civil War:
Little is known about the black men and women who served as Union intelligence officers, other than the fact that some were former slaves or servants who escaped from their masters and others were Northerners who volunteered to pose as slaves to spy on the Confederacy. There are scant references to their contributions in historical records, mainly because Union spymasters destroyed documents to shield them from Confederate soldiers and sympathizers during the war and vengeful whites afterward.
One whose story is remembered is Harriet Tubman, pictured above. "Often disguised as a field hand or poor farm wife, she led several spy missions into South Carolina while directing others from Union lines", Holland writes. Read the full story.
The Library of Congress has compiled a list of online resources about Harriet Tubman.