I'm not sure how to give a plot synopsis here -- basically, it's the continuation of the Trials of Apollo. He has another task to accomplish -- another of the new emperors to take down before the third one, in the next book. It's the same ol' set up that has served Riordan so well -- and will continue to do so for years to come.
Basically, Apollo/Lester has to go and find another Oracle. To do so, really, he has to face a lot of people that he's hurt/disappointed over the millennia. He learns a lot about himself, matures a bit. That part was good -- and the whole thing was entertaining. But it felt stale. I liked The Hidden Oracle a lot and was excited to see where this series went. Now, I'm not so sure. I'll finish the series, but with greatly diminished expectations.
Not that it got into details, but there was a lot more intimated/flat-out said Apollo's sexual history than I'm comfortable with for a MG book. The previous books in the Percy-verse suggested sexual orientation and activity, there was some romance, but this went much further than any of those. Honestly, it went a step too far. If this wasn't a part of the Percy-verse, or was clearly marketed toward older readers, it wouldn't have been that bad and I wouldn't have said anything about it. But that's not the case here.
As far as the audiobook goes, it was rough. Robbie Daymond was very aware that he was reading amusing material and he read it like each line was a punchline. It was the vocal equivalent of mugging for the camera, if you will. Now, there were a couple of serious and poignant moments, and Daymond pulled those off well, but otherwise it was tough to listen to.
I didn't like the narration, and didn't think the story/writing was as crisp as the first book in the series. But it was still entertaining enough. This isn't the one to start reading Riordan. But it'll do for his older readers.