The Chalice of the Gods (original) Rick Riordan

Percy Jackson, modern-day son of Poseidon, is just trying to get through high school. After saving the world multiple times by battling monsters, Titans, and giants, Percy is now settling in at Alternative High School in New York, where he hopes to finally have a normal senior year.

Unfortunately, the gods aren’t quite done with him yet. Poseidon breaks the bad news that if Percy expects to get into New Rome University, he will have to fulfill three quests in order to earn the necessary three letters of recommendation from Mount Olympus.