The Heroes of Olympus #3: The Mark of Athena by Rick Riordan
In The Son of Neptune, Percy, Hazel, and Frank met in Camp Jupiter, the Roman equivalent of Camp Halfblood, and traveled to the land beyond the gods to complete a dangerous quest. The third book in the Heroes of Olympus series will unite them with Jason, Piper, and Leo. But they number only six--who will complete the Prophecy of Seven?
The Greek and Roman demigods will have to cooperate in order to defeat the giants released by the Earth Mother, Gaea. Then they will have to sail together to the ancient land to find the Doors of Death. What exactly are the Doors of Death? Much of the prophecy remains a mystery. . . .With old friends and new friends joining forces, a marvelous ship, fearsome foes, and an exotic setting, The Mark of Athena is another unforgettable adventure by master storyteller Rick Riordan.