The protagonist, Felix the Fox, is a lower class Roman citizen who has managed to move up in the world by becoming a master of Numina (magic) but has fallen into disrepute due to his master's mental degeneration. Felix has become the first private detective in history, two thousand years before Sherlock Holmes, by marketing his services to wealthy clients with not entirely respectable problems.
In this story, Felix finds himself hired by a rich patrician who offers him double his usual fee in order to investigate the horrifying death of his son. The son was clearly killed by sorcery due to the fact his heart has become a literal ruby. Felix begins investigating the poor boy's relationship to a local street performer, a secret cult of revolutionaries, and his honey-producing cousins that stand to benefit from his death.
The occult detective is a tried and true formula in writing with John Constantine, Harry Dresden, and others all serving in various capacities over the years. Felix is an appropriately shady sort of fellow and is less concerned about justice than being paid for a successful investigation. Class is never disregarded in the story even if Felix is capable of maneuvering through it better than most Romans.
The book doesn't actually take place in Rome proper but a fantasy equivalent called Egretia. It is probably a good idea as this gives Assaph Mehr more freedom to incorporate whatever elements he wants in addition to magic. I should note that sometimes he does mention historical figures like Cicero that seem incongruous to the storyline. Despite that, the world is extremely believable with many elements drawn from archeological findings about the period.
I like the magical system of the world. Rather than simply have magical effects function by saying a few magical words, they have a complicated system of spirit evocation. There is also a debate whether the spirits are sentient or not. It is a good representation of the conflict between the Roman philosophers of the time and traditional religion.
Overall, I very much enjoyed the twists and turns in this book. Felix is an incredibly effective detective but has to run down many false leads and red herrings before getting to a genuine clue. Assaph Mehr's love of trivialities and culture is (mostly) accurate to the period. You get a real sense of how complex and fascinating Roman society was. I so enjoyed this book I immediately bought the sequel and liked it even more.