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Cantor, Norman F. (1993). The Civilization of the Middle Ages. HarperCollins

Cowdrey, H.E.J. (1998). Pope Gregory VII, 1073–1085. Oxford University Press.

Jolly, Karen Louise. (1997). Tradition & Diversity: Christianity in a World Context to 1500. ME Sharpe.

McCarthy, T. J. H. (2014). Chronicles of the Investiture Contest: Frutolf of Michelsberg and his continuators. Manchester: Manchester Medieval Sources. ISBN 978-0-7190-8470-6.

Metz, René. (1960). What Is Canon Law? Hawthorn Books. New York.

Tellenbach, Gerd (1993). The Western Church from the Tenth to the Early Twelfth Century. Cambridge University Press.