The Diamond Of Darkhold by Jeanne DuPrau
The fourth book in the Ember series: better than People Of Sparks, better than Prophet Of Yonwood, not as good as City Of Ember. Like City Of Ember, it excellently captures that feeling you have as a child of wanting to go off on an adventure – but one not far from home, without too much danger, more fun than scary, like running away as you imagined it would be and not as it actually is.
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.