The City of Ember
by Jeanne DuPrau
Just over 200 years before the start of the book, The Builders write instructions for the citizens of Ember about how to escape their world. Friends Lina and Doon know that the increasingly common blackouts and supply shortages mean that they urgently need to find a way to save their city where all light comes from light bulbs and messages are carried across town by people. Lina stumbles across fragments of an ancient message and shares it with Doon. Together they risk their lives to find out what is past the Unknown Regions and to move into the future.