A fearless author who was blacklisted by the House Un-American Activities Committee, Marks published about twenty books on politics and religion, one of which received accolades from Arnold Toynbee and Herbert Marcuse. His first book, The Bear That Walks Like a Man: A Diplomatic and Military Analysis of Soviet Russia (1943), was reviewed in over thirty newspapers and received glowing praise from John Cudahy, FDR's former ambassador to Poland and Belgium. While researching his book on Russia, Marks was assisted by Secretary of State Cordell Hull, "the father of the United Nations," who gave Marks direct access to State Department files. In 1973 the JFK Library contacted him with a request to purchase Murder Most Foul!, his first nonfiction book on the JFK case. And in 1979 the House of Representatives Select Subcommittee on Assassinations cited five of Marks' titles in its report.