The Horla by Guy de Maupassant

Guy de Maupassant wrote this novella about a man's decent into madness, three ways, all of which are included in the book. They each tell the story of the man's struggle against what should not be and how his mind manages to explain and deal with it.
The book was published shortly before de Maupassant was, himself, institutionalised for insanity so could well be informed by his own experience.
It is a chilling and fascinating read and I appreciated the insight into what it might be like to lose your mind.