The Sea of Monsters: Percy Jackson and the Olympians Series, Book 2
by Rick Riordan
After a summer spent trying to prevent a catastrophic war among the Greek gods, Percy Jackson finds his seventh-grade school year unnervingly quiet. His biggest problem is dealing with his new friend, Tyson – a six-foot-three, mentally challenged homeless kid who follows Percy everywhere, making it hard for Percy to have any ‘normal’ friends. But things don’t stay quiet for long. Percy soon discovers there is trouble at Camp Half-Blood: the magical borders which protect Half-Blood Hill have been poisoned by a mysterious enemy, and the only safe haven for demigods is on the verge of being overrun by mythological monsters.