I read all four City of Ember books. They were pretty enjoyable; I'm curious to see the movie that they made out of the first one. Books one two and four all form a continuous narrative. The third book feels very disjointed, mostly due to fact that it is a prequel to the series. However, it does fit nicely into the narrative. I was thrown by it, but I don't think I would have wanted to read them in another order.
They are young adult fiction books. The three are about two teens in the dying city of Ember who want to change things and save the city. (And what happens next...)
The setting is interesting and the society is believable.