The Diamond of Darkhold (Ember, Bk.4)

Lina and Doon escaped the dying city of Ember and led their people to the town of Sparks. But they soon discover that winter is harsh above ground. When Doon finds a book with torn pages that hints at a mysterious device from the Builders, it doesn’t take much for him to convince Lina to join him for one last adventure in the city of Ember. But what - and who - will they find when they return?