Delaney, John J., and James E. Tobin. Dictionary of Catholic Biography. New York, NY: Doubleday, 1961.
Giesebrecht, Wilhelm von & Simson, Bernhard von, Geschichte der deutschen Kaiserzeit VI, Leipzig: Duncker & Humblot, 1895, available at the Internet Archive here: Volume VI. (in German)
Kelly, J. N. The Oxford Dictionary of Popes. New York: Oxford UP, 1986.
Levillain, Philippe, ed. The Papacy: An Encyclopedia. New York: Routledge, 2002.
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