My 12-year-old daughter introduced my wife and me to the immensely popular Attack on Titan TV animated series (it's on Netflix). We quickly tore through all 25 episodes and loved it in the same way we love zombie movies, Game of Thrones, and Guillermo del Toro's The Strain.
In short, the story takes place in a future where the human race has been nearly wiped out by giant humanoids who eat people for fun. The remaining humans live in land enclosed by three tall concentric walls. They have no electricity or gasoline engines, and are constantly being attacked by the titans, who seem incapable of (or unwilling to) communicate with people.
Now that we must wait for the second season (and the upcoming live action movie), we've started reading the manga, which came before the TV series, and extends way beyond where the first season of the TV show left off. Amazon has a good deal on the first Attack on Titan anthology, which collects issues 1-4 of the Manga. It's just $(removed) and runs 208 pages. I know I'm late to the party, here — I imagine a lot of you are familiar with the show, but if you have not, you are in for a treat.
Note: Attack on Titan is gory and intense. I think 12 is good age to start with the series. I wouldn't give it to a younger child.