Percy Jackson and the Last Olympian
Rick Riordan
Penguin Books Ltd

Most people get presents on their sixteenth birthday. Percy gets a prophecy that could save or destroy the world. That's how it goes when you're the son of Poseidon, God of the Sea. According to an ancient prophecy, bad things will happen when Percy turns sixteen - because he's the one who gets to decide the fate of the entire world. But no pressure.

Also in the same series:
Percy Jackson and the Lightning Thief
Percy Jackson and the Sea of Monsters
Percy Jackson and the Titan's Curse
Percy Jackson and the Battle of the Labyrinth
Percy Jackson: The Demigod Files