These MS books were also called horae (Lat. = hours). They were variations of the *breviary and used mostly for private devotions. The central text, the Little Office of the Blessed Virgin, was a shorter version of the devotions performed in the eight *canonical hours. The text, known from the 10c, entered into lay use by the end of the 12c, often attached to the psalter. Owned mostly by the nobility of the 14c, these books were often illuminated in a highly elaborate and expensive fashion. The best-known such work is perhaps the Tres Riches Heures du Due de Berry, produced in Flanders by the Limbourg brothers c.1416 (but finished later).
Dictionary of Medieval Terms and Phrases. Christopher Coredon with Ann Williams.