The Hidden Oracle by Rick Riordan
My name is Apollo.
I used to be a God.
But that was before I was cast down from Olympus (a terrible misunderstanding) and woke up in a New York dumpster.
I persuaded Percy Jackson to give me a lift to Camp Half-Blood. But the place is in chaos. Demigods are going missing and the all-seeing Oracles have fallen to a mysterious enemy known only as 'the Beast'.
I figured that regaining Zeus's favour would involve a series of harrowing trials, and I was right (I usually am). My first task? Protect the final Oracle, hidden away for millennia. If I fail, the Beast will control the future, but worse - I'll be stuck as a teenage boy, forever...