Shift is the second book in Hugh Howey's "Silo" trilogy. The first was Wool, which I liked a lot. This one, not so much. (And you might want to stop reading if you haven't read Wool, because what follows could be considered spoilerish.)
In Wool we were introduced to the dystopian world of the silo, a huge cylinder of metal and concrete, where humanity was relegated into a totalitarian system, unbenignly controlled by a few shadowy folks. The external world (which turned out to be just outside Atlanta) has been transformed into a hellish nightmare. The big reveal at the end (unveiled by a plucky young heroine) was that there were many silos, and another shadowy level of control over them all.
Shift answers the question: how did that happen? In the near future, technology has given us the ability to design nanotech machines that can save our lives; but also quickly destroy them. A young Congressman is recruited on the pretense of designing a shelter to protect workers at a new facility for handling radioactive waste. But the true purpose is soon revealed.
The book also follows the backstory of "Solo", who was encountered in the first book. He didn't have an easy life.
The book has a high rating on Amazon, so your mileage may vary, but I found it obscenely overwritten, as if the author had a word count in mind, and just kept stuffing sentences into chapters until his goal was met. The plot isn't heavy enough, and the characters are not complex enough, to deserve all those words. The characters undergo a lot of anguish, mental and physical. It's all painfully described.
Also, I read Wool back in 2015, and maybe shouldn't have waited so long to get to this one. It's easy to forget what you're supposed to make connections with.