Song Of Achilles by Madeline Miller

The book is narrated by Patroclus, the son of King Menoetius. He is presented as a potential suitor to Helen of Troy. He is then obliged to take a blood oath in defense of her marriage to Menelaus. After Patroclus accidentally kills the son of one of his father's nobles, he is exiled to Phthia where he meets Achilles, the son of Phthia's king Peleus and the sea nymph Thetis. They become close friends and Patroclus develops feelings for Achilles. Convinced that a mortal of low status is an unsuitable companion for her son, Thetis attempts to separate the pair by sending Achilles to train under Chiron for what would become three years, though Patroclus ultimately joins Achilles in his training, having run away and caught up to Achilles before he even meets Chiron. As their relationship grows, the Mycenaean king Agamemnon calls on the various Achaeans to join his military campaign against Troy, whose prince Paris has kidnapped his brother Menelaus' wife Helen. As a prophecy foretells that Achilles will die in Troy after the death of the Trojan prince Hector, Thetis hides Achilles on Skyros in the guise of a woman in the court of King Lycomedes; she forces him to marry Lycomedes' daughter Deidamia, who later bears Achilles' son Neoptolemus. Patroclus follows Achilles to Skyros, where they live until they are discovered by Odysseus and Diomedes. Patroclus is obligated to join the war in Troy as a result of his blood oath, while Achilles joins after vowing that he will never fight