The House of Hades

At the conclusion of The Mark of Athena, Annabeth and Percy tumbled into a pit leading straight to the Underworld. The other five demigods have to put aside their grief and follow Percy's instructions to find the mortal side of the Doors of Death. If they can fight their way through Gaea's forces, and Percy and Annabeth can survive the House of Hades, then the Seven will be able to seal the Doors from both sides and prevent the giants from raising Gaea. But if the Doors are sealed, how will Percy and Annabeth escape?
Series Information: A prophecy has decreed that seven demigods - half mortal, half god - will defend the world from the earth goddess Gaia, who is out for revenge against the Olympian gods. But there's a catch: some of the demigods are Greek and some are Roman, and these two groups are longtime enemies. They'll have to unite to put a stop to Gaia and her army of giants and monsters! Bringing the teen heroes of Percy Jackson & the Olympians together with a whole new cast of characters, author Rick Riordan has created an outstandingly fun adventure deeply rooted in mythology that will satisfy both longtime fans and new readers. Combining nonstop action with the unmatched drama of the gods, the books will have readers racing to turn the pages, and seeking out the original stories.