This story happens after The Heroes of Olympus series. You don't necessarily have to read Percy Jackson and The Olympians and The Heroes of Olympus first, but I would highly recommend it. It is a continuation of that storyline. Percy, Nico and some of the other characters pop up a few times. Camp Half-Blood is visited. Several previous characters are mentioned. Events that happened at the end of Heroes of Olympus are what causes the situation in The Hidden Oracle. Because of what went down with the gods, Apollo is being punished and the oracle is missing. Apollo has been turned human, and he has to complete a quest. I like both series mentioned above and Kane Chronicles series, but I did not like The Sword of Summer so I was worried about this one. If you liked the other Greek/Roman books, you will probably like this one. It has the same humor, action, mystery, sacrifice and family problems.