The City of Ember by Jeanne DuPrau

It is always night in the city of Ember. But there is no moon, no stars. The only light during the regular twelve hours of "day" comes from floodlamps that cast a yellowish glow over the streets of the city. Beyond are the pitch-black Unknown Regions, which no one has ever explored because an understanding of fire and electricity has been lost, and with it the idea of a Moveable Light. "Besides," they tell each other, "there is nowhere but here" Among the many other things the people of Ember have forgotten is their past and a direction for their future. For 250 years they have lived pleasantly, because there has been plenty of everything in the vast storerooms. But now there are more and more empty shelves--and more and more times when the lights flicker and go out, leaving them in terrifying blackness for long minutes. What will happen when the generator finally fails? Twelve-year-old Doon Harrow and Lina Mayfleet seem to be the only people who are worried. They have just been assigned their life jobs--Lina as a messenger, which leads her to knowledge of some unsettling secrets, and Doon as a Pipeworker, repairing the plumbing in the tunnels under the city where a river roars through the darkness. But when Lina finds a very old paper with enigmatic "Instructions for Egress," they use the advantages of their jobs to begin to puzzle out the frightening and dangerous way to the city of light of which Lina has dreamed.