The City of Ember: Jeanne DuPrau

In the future, some of the last remaining humans on earth are moved to an underground city. When Lina finds the clues to escaping the town that she grew up in and the town that is running out of supplies, she decides to follow the list of clues to safety. Lina, Doon, and Lina's little sister go on an extremely risky journey to discover the earth that they came from, not the one underground.