Short answers, pulled from the story.
A Book of hours is a type of Christian devotional book used by lay people to pray the canonical hours from Matins to Compline. These books were the most common type of surviving medieval illuminated manuscript and often served as the only book in a household for teaching children to read.
In the 14th century the Book of hours overtook the psalter as the most common vehicle for lavish illumination. This shift reflected the increasing dominance of illumination commissioned and executed by laymen rather than monastic clergy.
The French prince John, Duke of Berry (1340, 1416) owned the Très Riches Heures du Duc de Berry. This manuscript was begun around 1410 by the Limbourg brothers and left incomplete by them before decoration continued over several decades by other artists and owners.
Black Hours are codices copied on black pages that used gold and silver ink to create a shimmering effect against the dark background. This striking variation emerged in the late 15th century and was reserved for the most powerful and wealthy patrons who could afford the time-consuming process of creating such a manuscript.
Calendar pages in a Book of hours depicted the twelve months of the year with specific labors or activities and zodiac signs linking human experience to the cosmic order. These pages were used to record important dates and events, teach children to read, and personalize the book with localized saints and local festivities.
By the end of the 15th century the arrival of printing made books more affordable and much of the emerging middle class could afford to buy a printed Book of hours. The market contracted sharply by 1500 and the finest quality books were once again being produced only for royal or very grand collectors.