As historical novels go, it doesn't get too much better than this adroit little tale. Personally, I find Renault's dry language a bit off-putting at times, but I know that others will find her style much more agreeable. It depends on personal taste. I find it slightly stuffy, others will see it as rich and evocative.
Anyway, this book takes place during a particularly fascinating era in Ancient Greek history, the time of the Peloponnesian War. The city of Athens had reached its highest point and after the war it would never again be what it was. The characters in this book do not know that however, and that's what makes the story so bittersweet and compelling.
Bonus appearances from Socrates and his associates provide extra flavor for those who like a bit of philosophy in their fiction. Others will be annoyed by the pontificatory tangents. Once again, it's a matter of taste.
Also, since this is a Renault book, you should know that the romantic love of two young men is a central plot point. I personally find it refreshingly unconventional. (It certainly was refreshingly unconventional in 1956 when this novel was first published! Mary Renault deserves approbation for being a trailblazer in LGBT fiction, but if you have problems with the subject you can skip it. You will be missing out on top-notch historical fiction though.)