Columbia subject headings:
Harvard subject headings:
Espionage--Germany--Berlin--History--20th century.
Intelligence service--Germany--Berlin--History--20th century.
Spies--Germany--Berlin--History--20th century.
Princeton subject headings:
Espionage--Germany--Berlin--History--20th century.
Intelligence service--Germany--Berlin--History--20th century.
Spies--Germany--Berlin--History--20th century.