The Song of Achilles by Madeline Miller

We are in Ancient Greece, a young boy Patroclus fails to live up to the hope of his father the king and finds himself exiled for being too weak to defend himself after accidentally killing another boy. His weight in gold pays for his place at the court of another king, and it is whilst trying to make himself invisible that Patroclus comes to the attention of golden child and prince Achilles. Deemed by prophecy to be unbeatable in battle, Achilles is the child of the king and a sea nymph. The story follows the boys as their friendship deepens, through their time studying medicine, astronomy and hunting with the centaur Chiron to the Trojan War and the events of The Iliad. 

I was captivated, the story was well told and the fantastical elements of gods walking with mortals, begetting children and influencing events never seemed like whimsical fantasy, they were as real as the other characters.