Full Dark, No Stars, by Stephen King

This book is a little dark jewel. It has 4 stories that explore the concept of "a man inside a man", a darker part of oneself that's capable of doing things you never know you were capable of. Triggered by exceptional circumstances, it shows that every man and woman is capable of doing terrible things.
In the first story, 1922, a farmer kills his wife in isolated Nebraska, aided by his 14 year old son, to prevent her from selling some land and destroying their farm. But the dead have a way to get their payback...
In the second story, Big Driver, a mystery novel writer discovers she's capable of murder after being raped and left for dead in a deserted road.
In the third one, Extensions, a man faced with his own mortality makes a deal with the devil, condemning his hated/best friend to a fate worse than death.
And in the last one, a wife discovers an ugly truth about her husband of 20 years, and decides to take matters in her own hands...
Every protagonist is a regular, ordinary man or woman, who becomes capable of doing unthinkable things due to extraordinary circumstances. I liked this concept very much, and I think King explored it beautifully.