Sebastian Piotr Bartos, Negotiations of Power in a Medieval Society: Ecclesiastical Authority and Secular Rulership in Little Poland, 1177-1320,ProQuest, 2008. page 165.
Anne Duggan, Nobles and Nobility in Medieval Europe: Concepts, Origins, Transformations. Boydell & Brewer Ltd,2000 page 124 & 103.
Richard W. Unger, Jakub Basista, Britain and Poland-Lithuania: Contact and Comparison from the Middle Ages.BRILL, 2008 page 72.