The Son of Neptune (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.