Until 2008 the Jaharis Lectionary was a hidden treasure: an illuminated Byzantine manuscript that was almost entirely unknown, even to scholars. Superbly preserved, it is arguably the most important Byzantine work to come to the Metropolitan Museum's renowned collection since the 1917 gifts of J. Pierpont Morgan.  It represents the apogee of Constantinopolitan craftsmanship around the year 1100. In this important study, John Lowden, a leading expert on Byzantine manuscripts, discusses his discoveries about this extraordinary manuscript within the broader context of Byzantine book illumination. He traces the book's history from its acquisition to its production in Constantinople. By detailed analysis and comparison, the author shows how the manuscript was made for use in the patriarchal church of Hagia Sophia.