Wool by Hugh Howey

I really loved this sci fi novel. It's exciting and intelligent too with some great characters. It's the first of a trilogy, the second one being Shift. I enjoyed that one just as much. Apparently, the last one will be out in a couple of months. The story involves the world a few hundred years into the future and the characters live underground in a vast multi levelled silo. What happened to the Earth and why they're living in a silo comes out over the course of reading Wool and Shift. This knowledge though brings great danger for the characters who try to find out if there is life beyond the silo.