Percy Jackson has had an unnervingly quiet school year. But then he discovers that the magical borders protecting Half Blood Hill are failing. Unless something is done, the camp will be attacked by demons and monsters. The only way to restore power to the camp's borders is to find the mythical Golden Fleece. And the only person who can undertake this dangerous quest is Percy...

Also in the same series:
Percy Jackson and the Lightning Thief
Percy Jackson and the Titan’s Curse
Percy Jackson and the Battle of the Labyrinth
Percy Jackson and the Last Olympian
Percy Jackson: The Demigod Files