I enjoyed this book. It was hard to put down, especially in the last half when I stayed up until 2:30 in the morning reading it. It's hard to pin down exactly what I liked, and I often have that trouble with book reviews, but it's much easier to write what I didn't like. 
 I was displeased in the beginning at how many things suddenly changed, such as (view spoiler)[Sheriff Holston dying, then Mayor Jahns dying, then Deputy Marsh dying as well because it didn't allow me to get attached to any of those characters. (hide spoiler)] I've learned since that the book was originally written as 6 short stories, so at least those sudden shifts in character focus make sense as the end of short stories even if they're annoying in long story form. 
 I didn't like that none of the cleaners ever (view spoiler)[chose not to clean, for hundreds of years. It seems more like they would just leave, because the people who sent them out were trying to kill them. Maybe they'd try to scratch a message in the dirt, or send some other kind of signal to the people still in. It doesn't seem like they would all just clean, anyway. (hide spoiler)] 
 It was good that towards the end Bernard started to develop more depth than just as a cackling villain, with (view spoiler)[his explanation of the heavy responsibilities of his job and how he didn't want to do it and hated the people responsible for starting it all, although he does shortly after become just a bad guy again and is then killed by the author. (hide spoiler)] 
 I also don't like the name. Why 'Wool'? Surely 'Silo' is a much better title, as I thought to myself about a hundred times throughout the book.