MLA

Giles, of Rome, Archbishop of Bourges, approximately 1243-1316. De regimine principum. [between 1300 and 1325]. https://collections.library.yale.edu/catalog/11403242.

APA, 6th edition

Giles, of Rome, Archbishop of Bourges, approximately 1243-1316. ([between 1300 and 1325])[De regimine principum]. https://collections.library.yale.edu/catalog/11403242.