Second part in the Silo trilogy, this really is the prequel to Wool. Here mysteries from the Wool universe are explained. It contains the necessary background information to make Dust, the last part in the trilogy, possible. That said, it is a great book, with great characters, some known and some new. This part expands on the dystopian universe making it bigger and more detailed.
As usual in Hugh Howey's books, it is really well written, there are multi-dimensional characters, and plot twists that keep you at the edge of your seat.
In my opinion it's not as good as Wool, but this is probably because Wool was the first, and novelty is usually more striking in fiction. However this is not to say that I was disappointed. My expectations were thoroughly met, and I was happily satisfied.
I usually read other works between parts of a series, so as not to get "saturated" with the world or characters; however in this case I was unable to follow my own rules and had to read Dust as soon as I finished Shift.
My honest opinion: one of the greatest sci-fi dystopian books I've read in the last years.