So this completes the reading mini-project I set for myself when I read Hugh Howey's Wool back in 2015: to read the remaining members of the trilogy. Number two was Shift, which I read back in August 2017. Overall, I liked Wool a lot, Shift not so much, and Dust even less. But it was a worthy effort. As always, what follows could be considered spoilerish.
Another lesson: don't let too much time elapse between reading books in a strongly plot-connected series; I found myself wondering: Um, who are these people again, and what did they do in the previous books? I eventually remembered enough to make sense out of the plot here.
Basically: inhabitants of the silos have come to a dim realization of their reality. They're buried in deep Georgian soil, the outside world might be uninhabitable (or it might not), the folks running the show are only slightly more aware of the situation, and the real (cartoonish) bad guy has no compunctions whatsoever about mass murder.
As we open, the inhabitants of Silo 18, led by the intrepid Juliet, are using their newly-discovered driller to tunnel over to Silo 17. This isn't easy, and there's plenty of opposition. And (as we know) Silo 18's population is far from saintlike. And (as we also know) if Silo 1 gets wind of these doings, they'll wipe out both 17 and 18. So everything's fraught with danger, physical and mental despair, and also sentimentality. (A significant subplot: a cute kid loses her puppy—conveniently named "Puppy"—and gets involved with some nasty people instead.)
The problem I had with Shift is more-than-slightly magnified here: too many words to support a thin plot and characters.