The Hidden Oracle is an odd book. As a second sequel series to first Percy Jackson and the Olympians and then the Heroes of Olympus, we do get new main characters characters: Apollo, now in mortal form and Meg, surprisingly powerful plant magic demigod with some cool Roman swords. But we also get a number of guest appearances and references to previous characters: especially Percy Jackson himself.
It’s interesting have a setting and characters we can relate to to help ground us in the new series, but on the other hand, if I wanted to read about Percy Jackson, couldn’t I just read his series again? I remember having the same problem with Heroes of Olympus.