The City of Ember
We've got your back. With the Tough-O-Meter, you'll know whether to bring extra layers or Swiss army knives as you summit the literary mountain. (10 = Toughest)
(4) Base Camp
Recommended for kids nine and up, The City of Ember is written in an easy-to-understand way. The two protagonists are both kids in the 12-ish range, and since we see things through their eyes, we see a simplified version of the world. Then again, Ember itself is also like a simplified version of the world: there's no gun violence because there are no guns, and no war because there aren't divisions based on ethnicity or religion in their society.
Sure, when you actually start thinking about the story's implications, there are plenty of interesting and complex ideas there, but the story is written in a straightforward and engaging way, so we're guessing just about anyone can get into it and grasp the plot.