The Last Boeotian (Percy Jackson)

Everybody knows the story of Camp Half-Blood. Of seventeen year old Percy Jackson, of his deceased half-brother Tyson.
But do they know the story of the Euboea Base?

There have been increasing numbers of appearing monsters. People have been hurt and killed, mortals and the divine, and everything in between. The only ones brave enough to fight them are the demigods- with mortals not being able to see the monsters, and the gods hiding in Olympus, as always. Monsters once thought mythological, even to demigods, show themselves in places they shouldn't be.

Calixia is the only child of Aigaios, the god of the sea. She's never really known her family outside of the few people she knows in the base. When the base is split up and broken after a leviathan attack, who is she to meet but the demigods from another camp?

(Percy Jackson And The Boeotians: Book One)