MLA

Geoffrey, of Monmouth, Bishop of St. Asaph, 1100?-1154, creator. Historia regum Britanniae; with French moralistic poetry, etc.. between 1175 and 1250. https://collections.library.yale.edu/catalog/9998995.

APA, 6th edition

Geoffrey, of Monmouth, Bishop of St. Asaph, 1100?-1154, creator. (between 1175 and 1250)[Historia regum Britanniae; with French moralistic poetry, etc.]. https://collections.library.yale.edu/catalog/9998995.