As of now, Wool (Part One) by Hugh Howey is free to download on Amazon (US, ES). It's not the first time that this ebook is free, but I think I haven't posted about it before. This is the synopsis:
Thousands of them have lived underground. They've lived there so long, there are only legends about people living anywhere else. Such a life requires rules. Strict rules. There are things that must not be discussed. Like going outside. Never mention you might like going outside.
Or you'll get what you wish for.