Not a book that shouldn't be read, but a book about a book that shouldn't be read - Robert Chambers' "The King in Yellow", a fictional play that ends in madness.
"It is well known how the book spread like an infectious disease, from city to city, from continent to continent, barred out here, confiscated there, denounced by Press and pulpit, and censured even by the most advanced of literary anarchists. No definite principles had been violated in those wicked pages, no doctrine promulgated, no convictions outraged. It could not be judged by any known standard, yet, although it was acknowledged that the supreme note of art had been struck in The King in Yellow, all felt that human nature could not bear the strain, nor thrive on words in which the essence of purest poison lurked. The very banality and innocence of the first act only allowed the blow to fall afterward with more awful effect."
My theory about "The King in Yellow" is that it is supposed to be a summoning ritual in play form. Summoning rituals normally have three basic sections:
Preparation/cleansing
The Summoning itself
Banishing
But The King in Yellow is described as only having two acts. There is no banishing. And that is what drives people mad.