In the Conspiracy Theory Database elaborated by Karen Douglas for CREST* there are only refenced 16 books relevant to Conspiracy Theory before the 1970's and they seem to have missed Karl Poppe's 1945 book but referenced him for his 1966 second volume
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•The psychology of rumor , Allport 1947
•The authoritarian personality, Adorno 1950
•The Sociology of Georg Simmel, Simmel 1950
•The functions of social conflict, Coser 1956
•The Democratic and the Authoritarian State: Essays in Political and Legal Theory, Marcuse 1957
•The Semisovereign People: A Realist's View of Democracy in America, Schattschneider 1960
•Intergroup Conflict and Cooperation: The Robbers Cave Experiment, Sherif 1961
•Anti-intellectualism in American life, Hofstadter 1963
•The standard edition of the complete psychological works of Sigmund Freud, Freud 1964
•The paranoid style in American politics, and other essays, Hofstadter 1964
•Conflict and the Web of Group-Affiliations, Simmel 1964
•Considerations on the Causes of the Romans and Thier Decline, Montesquieu 1965
•The crisis of the early Italian renaissance: Civic humanism and republican liberty in an age of classicism and tyranny, Baron 1966
•The Open Society and Its Enemies, Vol. 2: The High Tide of Profecy: Hegel, Marx, and the Aftermath, Popper 1966
•The ideological origins of the American Revolution, Bailyn 1967
•The slave power conspiracy and the paranoid style, Davis 1969
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*CREST is the Centre for Research and Evidence on Security Threats and is “partly” intelligence funded by their own admission..