A Christian devotional book containing the prayers or offices to be said at the canonical hours of the day, especially popular in the later Middle Ages. The book of hours is an abbreviated form of the breviary, intended for lay people (those not in religious orders). It is the most common type of surviving medieval illuminated manuscript. Web resource here.
Jean Pucelle. The Hours of Jeanne d’Evreux. Folios 154v-155r. Ca. 1324–28. Metropolitan Museum of Art, New York.