This fits in between the mythic novels (The Bull from the Sea, The King must Die) and the Alexander trilogy, and is set a little after The Last of the Wine which was mid-C5th bce. It's now late C5th-early C4th bce and Nikerator is a tragic actor travelling Greece with his golden actor's mask of Apollo. He meets Dionysios of Sicily and witnesses his relationship with Plato and political experiments and failures to create not just an ideal republic, but the ideal philosopher-ruler.
Drenched in sunshine and full of an actor's anecdotes (this is really quite luvvie in parts!) together with backstage gossip about the Greek theatre, this is still steeped in the atmosphere of ancient Greece (or at least the hygienic one that we tend to want to culturally buy into).
Fans of Alexander will be rewarded by the glimpse of the boy who appears towards the end, and the failed hope of Plato that here at last was the right raw material for the development of the philosopher-king.
As elegiac as her other books I would guess this is less accessible since it's much quieter, more domestic, despite the political eruptions on Sicily. So read the 'big' books first (the Alexander series, Theseus duo) and then come back to this.