Percy Jackson and the Olympians Tome 2
The Sea of Monsters

Seventh grade has been surprisingly quiet for Percy ; not a single monster has set foot on his New York prep-school campus. But when an innocent game of dodgeball among Percy and his classmates turns into a death match against an ugly gang of cannibal giants, things get... well, ugly. And the unexpected arrival of Percy's friend Annabeth brings more bad news : the magical borders that protect Camp Half-Blood have been poisoned by a mysterious enemy, and unless a cure is found, the only safe haven for demigods will be destroyed.
In the follow-up to the wildly popular The Lightning Thief, The Graphic Novel, Percy and his friends must journey into the Sea of Monsters to save their camp. But first, Percy will discover a stunning new secret about his family - one that makes him question whether being claimed as Poseidon's son is an honor... or simply a cruel joke.