At a time when Charlemagne ruled, the Byzantines were encroaching upon Russia, and the faith of Allah was flourishing in Baghdad, there existed a kingdom with a tolerant, advanced civilization: somewhere between the Caucasus mountains and the Volga, the Khazar kingdom grew and flourished, and in one of the oddest choices ever made, converted itself to Judaism. A thousand years later, when the writer Marc Sofer is given an ancient Khazarian coin by...