A collection of Christian prayers for recitation at different times the day, books of hours were simplified versions of the eight periods of daily prayer observed by monks and nuns, from matins in the morning to compline at night. Written in Latin, they were the most popular type of religious book in medieval Europe. They vary somewhat in content and order (calendars, prayers, psalms, and masses for certain holy days were commonly included), as well as in the quality and extent of their decoration, which was customized to the patron.
One of the pinnacles of the genre is the Très Riches Heures of Jean, Duke of Berry. Like many of the best books of hours, this one was executed by Flemish artists, the Limbourg Brothers. The Limbourgs used a wide variety of colors obtained from minerals, plants, or chemicals. Among their more unusual colors were vert de flambe, a green obtained from crushed flowers mixed with massicot, and azur d'outreme, an ultramarine made from crushed lapis lazuli, used to paint the brilliant blues. This was, of course, extremely expensive. Luckily, cost was not an object for the Duke, who was an art connoisseur par excellence (with money gained from breaking the backs of the peasants, it must be said). His collections included jewels, castles, works of art, and exotic animals. His magnificent collection of books included astronomical treatises, mappa mondes, and a large number of religious books: 14 bibles, 16 psalters, 18 breviaries, 6 missals, and no fewer than 15 books of hours, including the Très Riches Heures. Filled with surpassingly beautiful miniature paintings and illuminations (called that because of the use of gold leaf), it represents the height of aristocratic bibliophily of the time.
Besides a facsimile reproduction of this art treasure, we have on hand Art of the Middle Ages, a volume of selections from The Hastings Hours (see the Annunciation, above) and notecards from The Bedford Hours ("The Flood" is shown below).
In addition to fauna and flora (as seen in the Hastings Annunciation), fantastical, grotesque, or whimsical elements often appear in the margins of medieval books, however serious in purpose. Here are two of my favorites, of the animal variety: