The House of Hades was an excellent book. Percy and Annabeth fell into Tartarus in book three, and now they are struggling to stay alive. They meet a titan that Percy pushed into the River Lethe, which brainwashes you. Now they are trusting that Bob won't remember his old self, and turn back to their enemy. Nico, Jason, Piper, Frank, Hazel, and Leo will try to close the Doors of Death on the mortal side. But now, the mortal side is buzzing with monsters.