City of Ember

The City of Ember, by Jeanne DuPrau
ISBN: 0385736282
Publisher: Yearling, 2008

Lina and Doon live in a city called Ember, which survives as an island in the darkness.  The people of Ember have never had contact with anyone outside the city.  As far as they know, they are the only people in the world.  Anyone reaching the edge of Ember will find themselves staring into darkness; anyone who goes into that darkness disappears or returns completely mad.  Lina and Doon, like all 12-year-olds, get their job assignments when they graduate from school.  Unlike other 12-year-olds, Lina and Doon use their new responsibilities--their access codes and contacts--to decipher a letter from Ember's founders.  Ember is dying.  It relies on a generator for all its power.  If the generator goes out, the whole city will be buried in darkness.  If Lina and Doon can uncover the founders' message, perhaps they can save their city from its fate.