Books of Hours – illuminated, Latin prayer books used for private devotion – were bestsellers in the Middle Ages. The ‘hours’ referred to divided the day into devotions (set prayers recited at a set time) known as the Divine Office. The eight offices (one every three hours) were: Matins (midnight); Lauds (dawn); Prime (6 am) Terse (9 am); Sext (noon); None (3 pm); Vespers (dusk); and, Compline (night, before retiring). Books of hours were used primarily by the middle and upper class laity, allowing them to immitate the monk by dedicating the day to prayer. On a practical level, books of hours were a source of art for the middle class.
A lovely 14th Century book of hours (that belonged to Jeanne d’Evreux) is on display at the Cloisters Museum in Manhattan (part of the Metropolitan Museum of Art). The Morgan Library and Museum in Manhattan also has a wonderful collection of books of hours.
Two very readable monographs on the subject are: The Hours of Richard III by Anne F. Sutton & Livia Visser-Fuchs (Alan Sutton Publishing Limited, 1996) and A Treasury of Hours by Fanny Fay-Sallois (Getty Publications, 2002). See also: The Hours of Catherine of Cleves (George Braziller, 1966).