Roma Eterna is a science fiction fixup novel by American writer Robert Silverberg, published in 2003, which presents an alternative history in which the Roman Empire survives to the present day. Each of the ten chapters was first published as a short story, six of them in Asimov's Science Fiction, between 1989 and 2003.
First edition
Agent of Byzantium is a 1987 collection of short stories by Harry Turtledove, centered on the exploits of Basil Argyros, a Byzantine secret agent. The stories are set in an alternate 14th century, where Islam never existed and the great ancient empires of Byzantium and Sassanid Persia survive.
First edition cover