I read the Iliad in high school but don't remember much. All I remember about Achilles is his arrogance and his vulnerable heel. Seen through the eyes of Patroclus, Achilles is less arrogant (at least at first) and more kind, honest, and without guile. Achilles learns about the prophesy that foretells that he will be the greatest among his generation at a young age. With such a future hanging over his head, he grows up alone even while surrounded by dozens of other boys. He is friendly to all the boys, but close to none of them, knowingly or unknowingly creating a loyal of group followers who will follow him into battle when the time comes.
Patroclus is in many ways the exact opposite of Achilles. He is awkward, not good at fighting or at any of the things a future king should be good at, and therefore is also a son who is unwanted by his father. When Patroclus commits an uncharacteristic act of violence, his father happily sends Patroclus away to be fostered by king Peleus, Achilles's father. Despite being shy, awkward, and adverse to anything resembling athletics, Patroclus catches Achilles's attention. First they are friends. Then they are more. Their relationship is both subtle and brightly lit.
Since the Iliad has been around for hundreds of years I don't feel like it is much of a spoiler to say lots of people die in the end, including Achilles. Miller sticks to the outline of the Iliad and so you know that Patroclus and Achilles are not going to spend their golden years together. Still the journey from beginning to end is golden to read about. Miller's clear and crisp writing makes the story even better. This is a very good read, even if you have no particular interest in Greek mythology.