The House of Hades
Greek and Roman demigods from the Prophecy of Seven must work together to seal the Doors of Death, and help Percy and Annabeth escape the Underworld in the process.
The House of Hades is the fourth book in the Heroes of Olympus series and the direct sequel to The Mark of Athena. At the end of the previous book Percy and Annabeth found themselves plunging into Tartarus, with the intention of finding the Doors of Death from the Underworld. Meanwhile the rest of the crew of the Argo II, Jason, Piper, Leo, Frank, Hazel, and an always sullen Nico De Angelo, must find their way to the Doors of Death in the mortal world: outside the House of Hades.
This entry in the Heroes of Olypmus series shows our main party divided, and even the bulk of that party, the aforementioned crew of the Argo II, often taking on adventures in groups of two and 3. This nicely mirrors the struggles of Annabeth and Percy in their trek through Tartarus. There are several old familiar faces we see from earlier on in this series as well as the earlier Percy Jackson and the Olympians series as a trip to Tartarus gives Riordan ample opportunity to re-introduce previously defeated enemies. One of the prime strengths of this entry is that all of the characters feel even more deeply realized and developed, a development that shows in everything from fart-jokes to the many high-pressure situations that occur throughout the novel.
Catch up on the Heroes of Olympus with The Mark of Athena