Editing Pope Gregory IX (section)

==Further reading==
* Pietro Balan, ''Storia di Gregorio IX e suoi tempi'' 3 volumes (Modena 1873).
* Kathleen Brady, ''[https://books.google.com/books?id=KN6OzgEACAAJ Francis and Clare The Struggles of the Saints of Assisi]''. (New York: Lodwin Press, 2021). {{isbn|978-1565482210}}.
* Joseph Felten, ''Papst Gregor IX''. (Freiburg i.B. 1886).
* Iben Fonnesberg-Schmidt, ''The Popes and the Baltic Crusades 1147–1254'' (Leiden, Brill. 2007) (The Northern World, 26).
* Guido Levi, ''Registri dei Cardinali Ugolino d' Ostia e Ottaviano degli Ubaldini'' (Roma 1890). 
* Damian J. Smith, ed. ''Pope Gregory IX (1227–1241): Power and Authority'' (Amsterdam University Press, 2023).
* Spence, Richard (1979). "Gregory IX's attempted expeditions to the Latin Empire of Constantinople: the crusade for the union of the Latin and Greek churches". ''Journal of Medieval History''. '''5''' (3): 163–176.
* Jeffrey M. Wayno. "[https://www.tandfonline.com/doi/full/10.1080/03044181.2022.2130963 Governing through influence at the thirteenth-century papal court]". ''Journal of Medieval History'' (2022).