Book 1 in the Varangian Series. "In the middle part of the Eleventh century, the Byzantine Empire dominated the world. The surviving part of the old Roman Empire, its position as the greatest known civilisation remained undisputed. Within the walls of its great capital city, Constantinople, treachery, debauchery and power politics dominated the lives of the ruling elite, as it had in the glory days of Rome."
Empress of the Seven Hills by Kate Quinn. Her third book about ancient Rome (the other two are related). She has a wonderful storytelling style, making the Roman people seem very alive and very real. Mistress of Rome should be read first, although you can still read this as a standalone.