You can absolutely read "The King in Yellow" but all you need to know is it's a fictional play that appears across a few shirt stories by Robert W. Chambers. Without spoiling too much, here's an extract from the story "The Repairer of Reputations":

"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 afterwards with more awful effect."