The Bedford Hours is considered by many to be the most sumptuous Book of Hours ever created by an illuminator’s hand—as well as one of the greatest treasures possessed by the British Library. It once belonged to John, Duke of Bedford, who was the younger brother of King Henry V and regent of France from 1422 until his death in 1435. Its 1,250 elaborate medallions, thirty-eight large-scale miniatures, and extensive brush gold, gold leaf, and silver...