The Son of Neptune by Rick Riordan (The Heroes of Olympus #2)
where he is. He spent the last few weeks getting trained by the wolf Lupa, who told him he was a demigod. Now he has to fight his way to the camp for half-bloods. But these kinds of things are never straightforward for demigods. The monsters keep attacking him and they seem to come back faster and faster each time.
He finally makes it, with a little help from Hazel and Frank, but the place feels all kinds of wrong.
Soon they have to go on a quest, to the land beyond Gods and face their own demons along the way. The danger awaits the heroes, new monsters, stronger, and deadlier than before and they don't stay dead.
The story is told from three different points of view. And Percy is finally back! :)