Medieval literature
Deyermond's published output was prodigious – 40 books, written or edited, and almost 200 articles ranging through four centuries of medieval Hispanic literature. He recognised that a comprehensive study of Medieval literature would require several books which exist for studies in English but which were lacking for Spanish. He particularly lamented the lack of complete dictionaries, bibliographies and historical syntheses; as a result he authored the medieval volume for the Ernest Benn History of Spanish Literature (1969), which addressed the lack of an historical synthesis.