I read and enjoyed the first book of the Silo Trilogy almost two years ago. Now I've finally read the second book (Shift) and I'm about to start the third (Dust). The most I can say about "Shift" without spoiling the story is that it is a prequel explaining how the world came to destruction and how the major characters of "Wool" reached the positions they have in that book. Again I was impressed with the careful planning and construction that Howey has put into the story and I'm eager to polish off the series.
Books - Shift, by Hugh Howey