The Yellow Wallpaper
I had never heard of this, but the writer and editor William Wiles recommended three stories on malign interiors. The others were The Dreams in The Witch House by HP Lovecraft and his own Care of Wooden Floors, and having enjoyed his book, I sought out and read the two short stories.
It probably will only take you twenty minutes to read this story, but it is twenty minutes that you are likely to remember for a very long time. It is a striking and memorable story, with plenty of telling detail dropped in, but still leaving some ambiguity.
A woman has given birth, and finds herself confined to an attic room, with a dreadful yellow wallpaper. The room has an unsettling effect on her. However the story is told with a real lightness of touch that draws you in, intriguing rather than grand guignol horror.
I would heartily recommend this to everyone, it is a perennial classic that deserves to be more widely known.