Summary: The character continue to get deeper into the mythology and danger.
This is essentially the 9th fully book (with a couple more short story collections) of the Percy Jackson series. This second series Rick Riordan has expanded his available mythology so that both Greek and Roman mythologies are available. I am far outside my own depth and knowledge at this point. That is not a problem because Riordan does not expect the reader to know everything. But there are more and more minor figures involved in every book.
At the end of the last book, Percy and Annabeth were sucked into Tartarus (the deepest place of the underworld where the Titans and other monsters get banished). The rest of the crew has to try to get to the door to the House of Hades to both let Percy and Annabeth out and to seal the door to keep the monsters from over running the world and to stop Gaia from waking up.
There is plenty of action for Percy and Annabeth in this book, but for the rest of the characters there is more character development than action. The action is there, but is mostly individual and focused on giving us back story of helping the characters see that they can do whatever they need to do to help the team and/or defeat Gaia.
The controversial part of the book (minor spoiler) is that one of the character reveals he has a same sex attraction to another character. Because there has always been underlying romantic themes in these book I am not much surprised. Recently it is part of a movement among young adult books dealing with gay character in younger and younger books. This is not nearly as central to the book as Every Day, but it is present and does explain some of the strangeness around the behavior of one of the characters in previous books.
While I enjoyed this book, it did feel like the series was dragging a bit. The back story may be important to the thrust of the series, but it feels like another book was added to encourage book sales, more than really drive the story.