As this is Womens History month, you may find The Civil War: Black American Contributions to Union Intelligence, a CIA publication, very informative. Of course it mentions Harriet Tubman, whose exploits are well known. It also describes the intelligence contributions of two other black women - Mary Touvestre, and Mary Elizabeth Bowser (picture).
Though interesting it is not comprehensive. Some other women who spied for the Union are
Sarah Emma Edmonds :
She disguised herself as a man to serve in the Union Army, and sometimes "disguised" herself as a woman -- or as a black man -- to spy on the Confederate troops.
Elizabeth Van Lew :
An abolitionist living in Richmond, she helped bring food and clothing to Union prisoners and smuggled out information. She also placed a spy in the home of Jefferson Davis.
Mary Edwards Walker :
Known for her unconventional dress - she often wore trousers and a man's coat - this pioneer physician worked for the Union Army as a nurse and spy while she waited for an official commission as a surgeon.
Mary Elizabeth Bowser :
She served as a maid in the Confederate White House -- and, ignored while important conversations were held, she passed along important information from those conversations and from papers she found.