In case you don't know, Animorphs is a sci-fi series targeted mainly at elementary/middle school students about five kids who are given the power to change into animals by an alien in order to fight a secret invasion of Earth by other, mind-controlling aliens. I read it back when I was ten years old, and a few weeks ago I picked up the first book at the library for nostalgia's sake. I was totally surprised to find that, instead of being a children's series, this is one of the darkest, most mature handlings of war that I've ever read.

Seriously, there's a book where a 13-year-old kid wrestles with the decision of whether or not to kill his own family (who have been taken by the aliens) in order to save himself and his friends, who are the only hope humanity has, but is unable to when the time comes, so his friends lose faith in him and marginalize him because they don't think he's ruthless enough for this war.

So, the survey:

What other children's books/series (And I mean other than Harry Potter ) surprised you by being well-written, enjoyable, and mature?