Percy Jackson & the Olympians: The Sea of Monsters

In the second book in the series, Percy Jackson finds out that Camp Half-Blood, the haven for demigods, has been taken over by monsters. Percy knows he needs his best friend, Grover's, help to save the camp, but he has been taken prison by the Cyclops Polythemus, in an island in the Sea of Monsters. After rescuing Grover, Percy and his friends set out to retrieve the Golden Fleece on the Island of the Cyclops to save Camp Half-Blood.