Once upon a time there was a girl who read a lot of books and wrote down what she thought about them. And sometimes she read really really dumb books, books way below her reading level (or interest), because her brother's place of employment, which was a bookstore, was looking for suggestions of young adult or children's books that were popular and not complete trash. So the girl thought, what the heck, all this reading, all this writing, all this world wide web, let's mash it together. So she did. That's all you get for explanation. Let's move along.
Wherein two youngsters aged 12 years who most 12-year-olds will have difficulty relating to improbably and unexpectedly find their way out of the city they live in. The boy, Doon, is intent on saving everyone from the inevitable darkness that will consume them when the city's generator dies and all the lights of Ember are forever extinguished. Lina isn't worried as much as Doon until she discovers the long-forgotten Instructions for Egress and realized they were meant all along to someday leave. The two children suddenly find themselves on the run from the authorities and are forced to escape using the Instructions, and after a float down the subterranean river, a climb towards the surface and a scramble through some tunnels, Surprise! The city you're from is underground, and there's a great big world up there!
Charming enough. Written, I think, for a younger age than I was expecting. There is nothing to prevent a child of any age with an advanced enough reading level from enjoying this book. The only difficulty is that children do not like to be wiser than the book they are reading. It takes away the magic to discover mistakes, or see solutions to simple problems that the characters (thus, apparently, the author) have blindly missed. The book encourages good morals, however, and the children are not bratty or superior to their elders as is so often presented in children's books. I'd recommend it for precocious young readers.