Heroes of Olympus, The, Book Four: House of Hades, The-(new cover) (The Heroes of Olympus) by Rick Riordan

The demigod heroes are now divided. While Percy Jackson and Annabeth Chase are trapped in the Underworld, the rest of the group—Jason Grace, Piper McLean, Leo Valdez, Hazel Levesque, and Frank Zhang—must navigate the treacherous lands of Greece to find the Doors of Death and free their friends. The heroes face challenges from the forces of the Titan king, Kronos, and battle against monstrous enemies. The tension heightens as the prophecy of the Seven nears its critical point, and the stakes are higher than ever. The group must deal with their personal struggles, including growing bonds and internal conflicts, while trying to prevent the ultimate war between the gods and Titans.