The City of Ember by Jeanne DuPrau

From chapters.ca:
The city of Ember was built as a last refuge for the human race. Two hundred years later, the great lamps that light the city are beginning to flicker. When Lina finds part of an ancient message, she’s sure it holds a secret that will save the city. She and her friend Doon must decipher the message before the lights go out on Ember forever! This stunning debut novel offers refreshingly clear writing and fascinating, original characters.