Improbable and Impossible SF discussion

I've just read this and loved it. Fast-paced action thriller page turner. People live underground in a huge silo, safe from the deadly environment on the surface. Some thoughts are forbidden, and to transgress means you get sent to the surface to die. People start to question how things are organised. Can't say any more without giving spoilers. Originally self-published as short stories and novellas, now redone as a trilogy of novels.