The Diamond Of Darkhold

Description for THE DIAMOND OF DARKHOLD by Jeanne DuPrau:

It's been several months since Lina and Doon escaped the dying city of Ember and, along with the rest of their people, joined the town of Sparks. Now, struggling through the harsh winter aboveground, they find an unusual book. Torn up and missing most of its pages, it alludes to a mysterious device from before the Disaster, which they believe is still in Ember. Together, Lina and Doon must go back underground to retrieve what was lost and bring light to a dark world.

This is #4 in Book Of Ember Series.