The Vienna native grew up in Berlin and became an anti-Nazi activist and underground publisher in Germany in 1933. He eventually fled to Stockholm where he founded a pro-Allies newspaper and a committee to free Nobel Peace Prize winner Carl von Ossietzky from a concentration camp.
He later became a spy and provided the Allies with information about Russian and Nazi activities, the Times said.
Singer left Sweden for the United States after it banned his biography of Hitler henchman Hermann Goering -- "Goering: Germany's Most Dangerous Man" -- in 1940. His next book was 1943's "Duel for the Northland: The War of Enemy Agents in Scandinavia."
The prolific author wrote many books about spies and crime and penned a variety of biographies ranging from actor Danny Kaye to President Lyndon B. Johnson, as well as children's books.
He founded the news service Singer Communications Inc. in Anaheim, Calif., in 1955 and served as its president.