The Son of Neptune (The Heroes of Olympus Book 2)
Percy is confused. When he awoke after his long sleep, he didn't know much more than his name. His brain-fuzz is lingering, even after the wolf Lupa told him he is a demigod and trained him to fight. Somehow Percy managed to make it to the camp for half-bloods, despite the fact that he had to continually kill monsters that, annoyingly, would not stay dead. But the camp doesn't ring any bells with him. Hazel is supposed to be dead. When she lived before, she didn't do a very good job of it. When the Voice took over her mother and commanded Hazel to use her "gift" for an evil purpose, Hazel couldn't say no. Now, because of her mistake, the future of the world is at risk.