The Sea of Monsters (Percy Jackson and the Olympians #2) by Rick Riordan

Percy Jackson’s seventh-grade year has been surprisingly quiet. 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 this fresh, funny, and hugely anticipated follow up to The Lightning Thief, Percy and his friends must journey into the Sea of Monsters to save their beloved 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.