Only 361 pages? Riordan is taking it easy on his readers. And maybe himself.
Anyway, following the events of The Heroes of Olympus, Zeus is a little displeased with Apollo and demonstrates this by turning him into a human teen (read: YA/MG novel star) and casting him to earth. He appears to be fully human -- not even a demigod like Percy and the rest. Speaking of Percy, as soon as Apollo figures out what happened to him and where he is, he makes a beeline for Percy's apartment to get help. Smart move. Percy gets him to Camp Halfblood and disappears back to NYC to do homework.
Once there, Apollo begins trying to figure out what quest he'll have to do to return his status to quo. Along the way, he'll make some friends, get a better perspective on himself and his offspring (yeah, that's not weird), and maybe go through some of that personal growth. Note that I said, "some" personal growth and "better" perspective -- that's not saying much, basically Apollo comes across as a teenaged-Gilderoy Lockheart with a conscience. Instead of the large number of missions that we've become accustomed to in these books, there's really just one (plus the series-arc mission) -- such a nice change.
A lot of people from the Percy Jackson and The Heroes series are name-dropped and discussed, not to mention the few that we see -- there's even a nod to the Magnus Chase series -- thankfully, my favorite is one of those who shows up in the flesh. There's also a good amount of in-jokes to please the long-time fans. But readers new to this universe shouldn't be put off by any of this -- it's absolutely approachable, maybe even moreso than anything since The Lightning Thief.
This is told in the typical breezy style that characterized non-adult mystery Riordan novels, but given the different protagonist, feels a little fresher. A little briefer, a little change of pace -- still full of that Riordan magic. The Hidden Oracle is a sold first-entry in yet another adventure in this world. Give it a shot.