Riordan did such a smart thing with this book, he took the two best-known characters and put them in Hades. Forcing Percy and Annabeth to strike out on their own, in a place where they couldn't get help from their fellow demigods, keeps them isolated and finally allows someone else to really shine. In a series where fans already "knew" who the hero was, shuffling characters around finally lets Riordan show off the heroic qualities in all his characters. And, he does an amazing job bringing Tartarus to life. His descriptions of the Underworld are better than of actual places he sends the other demigods to visit.
I know know why Riordan waited so long into the series to position his characters in this way. I knew he had to build out the story slowly since it's a long series with a lot of moving parts, but still, this is the first time I really felt like I was getting to know anybody. The narrative finally goes beyond simple back story for each demigod and in addition to all that we start learning about who they all really are and what they're experiencing on this quest, how it's changing them. In the midst of all this character development there is an actual mission the demigods much achieve - to reset the Doors of Death and prevent the monsters loyal to Gaea from quickly regenerating once defeated. The trick is in closing the Doors properly and they have to be accessed at the same time from both the entrance in Tartarus and the one in our world (located in the House of Hades in Greece.) Percy and Annabeth end up falling into Hades so they take on that part of the quest while the rest of the crew - Hazel, Piper, Frank, Leo, Jason, and Nico travel topside to the House of Hades. Everyone knows they're coming so there's no shortage of monsters trying to defeat the demigods at both ends of the quest.
As the journey continues the demigods are really given a chance to grow their talents. Frank evolves into a true son of Ares and a mighty warrior. It even changes his physical appearance. Hazel learns to manipulate The Mist, a new talent that really comes in handy in the major battles of this quest. Leo develops in a different, more personal way, but still changes for the better in a way that benefits the whole team. This particular quest really teaches the characters so much about themselves that I found this book to really be the most meaningful in the series.
Focusing less on Greek and Roman myths and more on the actual character of the characters makes House of Hades by far the best book I've read by Riordan since wrapping up Percy's own series. I'm excited to see where all these characters end up and am sad I've finally caught up to the publisher and now have to wait something like six months for the final book.
My review here is light on plot because I had a baby while reading this. Most of what I read before the baby was born got a little fuzzy after, but I definitely liked the book a lot and am still pretty happy with the series overall.