This is the sequel to City of Ember, and I liked it just as much. I think it has some thought-provoking ideas about what leads to war, and choosing the right thing even if it is hard. But keep in mind that it is a children's book, so you've probably thought the thoughts before. I'm an adult and I still really enjoyed it. I do think you should read the books in order to thoroughly appreciate them.
When the two civilizations come together, it is interesting what is of value to each. What they have and what they need. A little bit of brain versus brawn, but it's more complicated than that.
I liked the symbolism of the weather. The season's changing, the wind bring change.
Doon and Lina emerge the heroes again. There are some interesting new characters. Over all I just think its a great book. Probably for 9 and up.
Some quotes:
"Being good is hard, much harder than being bad."
"You don't want to, but you do it anyway."
"He hadn't really imagined what fighting would be like."
(Again I really liked Doon's father's simple wisdom.)
"She had hoped that someone else might to do, so she wouldn't have to, but nobody did."
