The People Of Sparks by Jeanne DuPrau

The citizens of the dead underground city of Ember emerge to the surface of a post-disaster Earth and try to make their way with the help of the people of a village called Sparks. Led by the young teenagers Doon Harrow and Lina Mayfleet, the 400 citizens of Ember walk several days after climbing from the cave where Ember was located. Tired and hungry, they come across an agricultural village called Sparks, which agrees to take in the Emberites for a time and provide them with food. The city of Ember have to help the city and earn their keep and maintain peace with the people of Sparks. This is a great sequel to The City Of Ember and carries on the tale.