The Lost Hero: The Heroes of Olympus, Book 1 by Rick Riordan (Paperback)

After saving Olympus from the evil Titan lord, Kronos, Percy and friends have rebuilt their beloved Camp Half-Blood, where the next generation of demigods must now prepare for a chilling prophecy of their own:

Seven half-bloods shall answer the call,
To storm or fire the world must fall.
An oath to keep with a final breath,
And foes bear arms to the Doors of Death