Harry Turtledove 
Agent of Byzantium 

In a Moslem-free universe where Constantinople never fell, the Byzantine Empire has not only survived but flourished, developing technology at an earlier date than in our universe. And spreading its power and influence throughout the world. But Byzantium has enemies who are jealous of its glory and would like nothing better than to bring it down and loot its treasures. Basil Argyros, Byzantium’s top agent, as his hands full, thwarting un-Byzantine plots and making the world safe for the Byzantine Empire.