Narratives of the lives (vitae) of the saints formed a popular genre from the early Middle Ages on. As holy figures continued to be canonized throughout the Middle Ages, new lives were composed and translations were made into the vernacular. Many of the early lives were incorporated into the Martyrology for use as readings in the Divine Office. Hagiography (the biography of saints) formed an essential part of the ecclesiastical library and was also a popular source for reading among the laity.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).