Imagine a 13th century Mediterranean and Middle East...
-Where the territories of western and southern Europe won back by the Romans during the 6th century reign of Justinian were not only maintained but expanded.
-Muhammad never developed Islam. Instead he converted to Christianity, becoming a holy man, and is now venerated as St. Moaumet.
In the absence of Islam's rise, both the Roman and Sassanid Persian (which has by now engulfed the entire Arabian Peninsula) empires remain as the two superpowers, existing in a sort of medieval cold war.
Into this world comes Basil Argyros, an agent of the Magistrianoi, the imperial secret police; sometimes he acts as a soldier, but more often he's a spy. During the course of his assignments as an agent of Imperial security, Basil also makes some exciting discoveries, thus making him an agent in another sense: as one who brings change and advancement to the Empire. From the Franks he steals a new weapon, recently cooked up by their monks--gunpowder. He returns from the lands of the Asiatic Jurchen nomads north of the Black Sea with an instrument we know as the telescope. He delivers to the emperor the secrets of printing, a recent Persian invention they've been using to foment insurrection in the Empire's eastern provinces. What perhaps is the most fascinating of all is Basil's witnessing the discovery of inoculation, made during a time of catastrophic plague in Constantinople.
Basil's nemesis in many of these stories is the beautiful and deviously clever Persian spy, Mirrane. As the two of them match wits, they develop a mutual respect and admiration, eventually falling deeply in love.
The Baen paperback edition contains the following seven stories:
"The Eyes of Argos"
"Pillar of Cloud, Pillar of Fire"
Only this edition contains the story "Pillar of Cloud, Pillar of Fire;" however, that story can also be found in Harry Turtledove's alt-history collection DEPARTURES (which also includes "Islands in the Sea," the story about Muhammad's aforementioned conversion to the Christian Faith.)
As someone with a Ph.D in Byzantine studies, Harry Turtledove knows the peoples and times upon which he bases this alternative world, making it a fun, fascinating read.