The Song of Achilles by Madeline Miller

Greece in the age of heroes. Awkward young prince Patroclus has been exiled to the court of King Peleus. Despite their differences, Peleus's golden boy, Achilles befriends the shamed prince. As they grow into young men, their bond blossoms into something deeper - despite the displeasure of Achille's mother, the sea goddess Thetis.

But when word comes that Helen of Sparta has been kidnapped, Achilles must go to war in distant Troy and fulfil his destiny. Torn between love and fear for his friend, Patroclus goes with him.