Master and God by Lindsey Davis.
Fans of Davis's Falco series will not want to miss her latest; a stand-alone novel of the emperor Domitian.
A talented tyrant and deluded despot, as he vents his suspicions, no one is safe. A reluctant hero; Clodianus is a brave man striving for decency in a world of corruption and deceit.
Flavia Lucilla hears the intimate secrets of the ruler who plays with the lives of his subjects as if he were indeed a careless god.
These two ordinary people must choose between their sworn duty to protect the emperor and an act of courage that will change the future of Rome.