Some of the medieval age's greatest treasures are its books of hours, virtually museums within hard covers. These Christian almanacs are crammed with prayers, psalms, and calendars of saints' days. They summarize all that medieval man worshiped and feared, depicted in miniatures wondrous with wildflowers and lavish with gilding. They are the unintentional encyclopedias of their era, and because each is unique, they are almost beyond acquisition.
When such a book does come along, its history is often shrouded in intrigue. Back in 1856, a Paris dealer sold a 193-page manuscript....