It was a great book to read and nicely wraps up the story arcs of Trials of Apollo characters and our beloved Percy Jackson and Heroes of Olympus characters.
One thing I didn't like about the ending was that the book closes of Apollo's story with him being conflicted between whether he wants to be a human or a God. It's great that he remembers Jason words of remembering to be human when he regains his Godhood but the ending showed him off as confused on the matter of him being a God again. He wants to be human but part of him wants to be a God too. So Apollo feels like he doesn't belong in either of the world's. Would have been nice if Riordan added a chapter or two more at the end which helped balance this out and showed Apollo returning to his old godly habits (but not as stupid as before).
The chapters could have showed him doing things with his powers after thinking them through (something he learned thanks to being a human). He looks after the children he sired stupidly as a God. But aside from being wiser and responsible (thanks to his time as a human) he also retains his old godly charisma. Makes jokes about Haphaestus, pulls pranks on Ares and Hermes. Gets into fights with Artemis. Is arrogant. Flirts with beautiful people (but doesn't sleep with them or persue them as lovers, cuz he's learned the lesson about that in his time as human)