The two kingdoms : ecclesiology in Carolingian political thought / by Karl Frederick Morrison. - Morrison, Karl Frederick. (OLS)http://pul/name/18668 (Princeton university press, 1964)
Bibliography: p. 271-97.

Die Humanitätsidee in der Geschichte des Abendlandes / Heinrich Hoffmann. - Hoffmann, Heinrich L. (OLS)http://pul/name/27364 (Lang, 1951)