The House of Hades (Heroes of Olympus, Book 4)

In 'The House of Hades', the fourth book of the 'Heroes of Olympus' series, Percy and Annabeth find themselves trapped in the Underworld and facing unspeakable challenges. Above ground, their friends race against time to find the mortal side of the Doors of Death to rescue the trapped couple and prevent monsters from invading the world. The demigods must face internal conflicts, gods, and personal fears to save each other and the world.