This particular book isn't really a novel. It's a collection of short stories about Basil Argyros, essentially a secret agent in 14th century Byzantine empire.
Except this Byzantine empire is a bit different than the one in our past due to a few minor changes:
1) An Arab named Moaumet, in the 6th century, takes a trading expedition to Syria, converts to Christianity and becomes a great saint.
2) There is never a great Schism. And Constantinople remains the center of *all* Christianity. There's a Bishop of Rome, but he's just one among equals.
One thing that I was acutely aware of was that Turtledove wrote these stories in the mid 80's.
I seriously doubt he could get them published now, due to 1). Back then, he was well known author yes, but well known in a particular genre, without media being so connected, and without the high sensitivity that is present now for anything that remotely touches anything near Islam. To publish an alternative history where Muhammad becomes Christian and Islam is never founded would be edgy today.
The book itself was fun. James Bond in an alternative Eastern Roman Empire. It was a nice read, since you could put it down between stories. While they were vaguely connected, each one could stand alone just fine. Oh, since Islam was never founded, the other main power besides Byzantium was Zoroastrian Persia. Basil does have his "Mary Sue" moments, when he is instrumental in discovering: the telescope, a cure for small pox, the printing press, and gun powder.
As I said, it's fun. A light read, and interesting if you like thinking about history from the "what if" perspective.
Sadly, it's out of print. But that's what used bookstores are for.