Series: Silo

Neat! Wool has a sequel. It's a pretty amazing story how Hugh self-published these and went to instant best-seller. 

Wait, I don't see Wool on my reading list...but I know I read the past.

Argh. Anyway, Shift gives a lot more backstory on the Silos and their creation / alleged purpose. It was a little nerve-wracking to go back and forth between two different stories, but that's also a good storytelling method. You want to hear more about a certain story, so you have to finish the next chapter to get back to your story. But then by the end of that chapter, you want to hear more of that story. And it goes on and on...