The House of Hades by Rick Riordan is the 4th book in the Heroes of Olympus series. In this book Leo, Piper, Jason, Nico, Frank, and Hazel have to get to the House of Hades so that they can close the Doors of Death. Meanwhile, Percy and Annabeth are trying to stay alive and sane in Tartarus. In order for the Doors of Death to be closed, you have to have a person on each side. So, Percy and Annabeth are also trying to reach the Doors to close them on the Tartarus side.
Very funny book, I really enjoyed it. Rick is very good at making sad books happy. (If that makes any sense.)
5 out of 5 stars.