Fast moving adventure in a parallel universe by the author of the 'Case of the Toxic Spell Dump'.
Unusually for Turtledove, there is only one protagonist in this book, unlike the large casts that he brings into his later works. The main character is a soldier who after an initial military intelligence operation is recruited into the Byzantine secret service, investigating Byzantium's potential enemies at home and abroad. In this alternative history, the biggest difference is that Mohammed became a Christian bishop instead of founding Islam, with the result that the two great world powers of the Middle Ages are Byzantium and the Persian Empire (both of which in "real" history were eventually overthrown by tribes owing their allegiance to Islam.
The agent, Basil, is something of a world-weary character, whose only affinity with certain other literary super-spies is the amount of times he happens to be in the right place to deal with trouble. Nevertheless this is a fascinating set of stories whose charm lies as much in the historical setting and the theology and science of the day as in the action.