Mark R. Cohen is Professor of Near Eastern Studies at Princeton University and a well-known authority on the Cairo Geniza and the history of the Jews in the medieval Islamic world. His books include "Jewish Self-Government in Medieval Egypt" (Princeton), which won the National Jewish Book Award for Jewish history in 1981; "Jewish Life in Medieval Egypt 641-1382," translated into Arabic, 1987; "The Autobiography of a Seventeenth-Century Venetian Rabbi: Leon Modena's Life of Judah"(Princeton); and, most recently, "Under Crescent and Cross: The Jews in the Middle Ages" (Princeton), which has been...