In 2015, a group of rare manuscripts from medieval monasteries went on exhibit at the New York Gallery of the international dealer Le Enluminures. The crown jewel of that collection was a book of the four gospels originally from an abbey in Germany. Viewers marveled at the ornate beauty of the 10th century volume—a crucified Christ carved in stunning relief on its oak cover, gorgeously illuminated chapter openings, and its text pages unfolding in elegant script.

But it was what was inside the cover that drew the attention of the authors of a new book, an intriguing circular diagram, "The Prayer Wheel."

Jana Riess holds degrees in religion from Wellesley College, Princeton Theological Seminary, and Columbia University. A senior columnist for Religion News Service, she is the author of "Flunking Sainthood: A Year of Breaking the Sabbath, Forgetting to Pray, and Still Loving My Neighbor."