City of the End: The Underwater City, Book 4: An Old Enemy
by Mark Mulle
After Lana saved her city and the Over World from Marketh, a man bent on controlling both worlds with his Ender Men army, the only thing she wants to do is relax.
Her plans are ruined, however, when she receives a letter that her friend, John, has gone missing. When Lana goes to meet with his sister, Ellie, to find the details, she finds herself sucked into another fight for her people.