Strange Case of Dr Jekyll and Mr Hyde by Robert Louis Stevenson (1886)

At a dark London corner a small girl is trampled by a man who simply walks on over her body, and the gentlemen who witness it come away sick with a loathing none of them can name. That nameless revulsion is the whole book's engine. Stevenson built what looks like a detective story around a secret nearly everyone now knows before opening it, that the monstrous Edward Hyde and the eminent Dr Jekyll share one set of bones, and then made the knowing worse rather than easier. The real horror is not that a good man kept an evil twin but that Hyde was never a stranger, he is the part of Jekyll that respectability required him to starve, and Jekyll, handed the chemistry to loose it, found to his shame that he preferred being free. Beneath the gaslit fog is a confession the Victorians could not make in daylight, that the self does not outgrow its hungers. It only locks them in a back room and listens to them breathe.