The Song of Achilles by Madeline Miller

The events leading up to the Iliad are told in a very personal way from the viewpoint of Patroclus, the longtime companion and lover of Achilles, in Madeline Miller's The Song of Achilles. Patroclus was only 9 when his father, King Menoitius, took him to present himself as a suitor to Helen, daughter of the Spartan king. (The same Helen who was the catalyst for the Trojan war.) When Patroclus was 10, he was disowned by his father,  and exiled to the kingdom of Phthia, where Prince Achilles chose him as a special friend.

Achilles' mother, Thetis, was a goddess. She took a strong dislike to Patroclus right from the start... and things do not go well when the gods are against you. Patroclus and Achilles know that they are doomed to die young. Despite this, their relationship is tender and strong. The ending is sweetly romantic, rather than tragic.

The Song of Achilles is currently on the shortlist for the Orange Prize (which also includes other books that I've reviewed: Half-Blood Blues, The Forgotten Waltz and State of Wonder).