The Diamond of Darkhold (The City of Ember #3) by Jeanne DuPrau
Lina and Doon are returning to the city of Ember, but what awaits them in the dark? This highly acclaimed adventure series is a modern-day classic—with over 4 MILLION copies sold!
Lina and Doon escaped the dying city of Ember and led their people to the town of Sparks. But it’s winter now, and the harsh realities of their new world have begun to set in. When Doon finds a book that hints at an important, long-lost device, it doesn’t take much for him to convince Lina to join him for one last adventure in the place they used to call home. But will this mysterious technology be enough to help their people? And what— and who—will they find when they return?