The Hidden Oracle is the first book of the series. It is told in first-person by the main character Apollo, and uses haiku for chapter titles. The book was released on May 3, 2016.

The book opens with Apollo landing in an alley in New York City—in the form of a mortal teenager named Lester Papadopoulos—and meeting Meg McCaffrey, a homeless daughter of Demeter. Believing that he must only undertake some simple quest to regain his immortality, Apollo/Lester and Meg travel to Camp Half-Blood, where they learn most of the world's oracles have stopped working. The two undertake a quest to locate and protect the one remaining oracle, hoping also to learn how to help the other oracles and to regain Apollo's immortality