The City of Ember (by Jeanne DuPrau)

Lina finds them, but not before her baby sister has chewed them into virtual indecipherability. Now she and her friend Doon must figure them out from the few disjointed letters left, before the city falls apart. Their efforts lead them to discover long forgotten secrets about the nature and purpose of Ember, and what the Builders intended. They want to share their discoveries with the rest of the citizens -- but only if the corrupt mayor and his guards don't stop them first.

Part dystopian fantasy, part mystery, part code-breaking treasure hunt, this is an intriguing story. Deciphering the clues from the damaged Instructions will keep readers guessing, despite some predictable plot twists in the center section, and they will be rooting for the appealing main characters.

While lacking the complexity or moral ambiguity of The Giver, THE CITY OF EMBER has its own unique features: It is a city designed not to be a utopia, but rather to hold the remnants of humanity safe and isolated for a fixed period, then release them. The problem comes when the release doesn't happen, no one knows that it should have, and the city is past its expiration date. A fascinating scenario --and one fraught with possibilities for the sequel.