Columbia subject headings:
Intelligence service--United States--History--19th century.
Spies--United States--Biography.
Spies--United States--History--19th century.
United States--History--Civil War, 1861-1865--Secret service.
Harvard subject headings:
Intelligence service--United States--History--19th century.
Spies--United States--Biography.
Spies--United States--History--19th century.
United States--History--Civil War, 1861-1865--Secret service.
Princeton subject headings:
Intelligence service--United States--History--19th century.
Spies--United States--Biography.
Spies--United States--History--19th century.
United States--History--Civil War, 1861-1865--Secret service.