Here are entered works by the Pope acting as an individual. For works by the Pope acting in his official capacity see also the heading, Catholic Church. Pope (1227-1241 : Gregory IX)
Catholic Church. Pope (1227-1241: Gregory IX). Decretales d. Gregorii Papae IX 1582.
Enc. brit. 15th ed. (Gregory IX, Pope, originally Ugo or Ugolino of Segni; b. before 1170, d. 8/22/1241)
New Cath. enc. (Gregory IX, Pope; b. Hugo [lino] at Anagui, c. 1170, descendent of the counts of Segni; d. Rome, 1241)
Wikipedia 4 April 2020: Pope Gregory IX (Latin: Gregorius IX; born Ugolino di Conti; c. 1145 or before 1170 – 22 August 1241) was Pope from 19 March 1227 to his death in 1241. He is known for issuing the Decretales and instituting the Papal Inquisition through his papal bull Ad abolendam, issued in 1184. He was born in Anagni and studies at the universities of Paris and Bologna. he was created a cadinal in 1198. and in 1220 made protector of the Franciscans. He was bishop of Osta 1206-1227. As pope he had long struggles with the Emperor Frederick II. He died in Rome.) https://en.wikipedia.org/wiki/Pope_Gregory_IX