The Books of Ember written by Jeanne DuPrau

This a series of 3 books.
the first being The City of Ember; people are living in the city. The city is running out of resources, some thing needs to be done to fix the problem before ember goes totally dark forever. The people of ember realize they are under ground in a 'shelter' kind of thing.
Once they are above ground that is where The People of Sparks begins. The people of Ember need a new life to begin over above the ground. The people of Sparks and Ember together create a 'new' village. The final book is The Prophet of Yonwood. In this book Crystal and her niece Nickie travel to Yonwood. There purpose was to clean out and sell the house Crystal's father lived in. The prophet supposedly is telling the citizens how to live, what to do because sinners are the problem. Nickie is receiving postcards from her father. He is on a job that is secretive. Her Dad is sending her clues where he is at, but what was he doing for all this time. In the end what does Nickie find out, Crystal does she move from NJ, does Grover graduate from high school. This last book in the series answers all your questions.
But wait I read the fourth book of Ember The Diamond Of Darkhold. In this last and final book Doon and Lina travel back to Ember. Doon was able to get a book from a roamer that sparked his interest in returning to Ember. Ember had stuff that the people of sparks could use to survive and also to trade but there was something else but what was it? Doon and Lina found this special item that changed the people of Sparks. Read the final book and revisit with Hoyt McCoy.