The King in Yellow is a medieval play infamous for inducing madness and despair among its readers, a monstrosity denunciated by pulpit and press, the one its author shot himself for bringing forth. 
 It is fictional. 
 This book contains concatenated weird macabre(and amazing) short stories, built around the above-mentioned infamous and forbidden play. 
 They are fictional too. 
 It would be probably safe to tag above collection as proto-lovecraftian since passing references to the Yellow King, Hastur and Lake of Hali often find their place in Cthulhu mythos, and further 'fear of the unknown' stories it inspired (including True Detective). 
 Fictional again. 
 Book in a book in a book. 
 Bookception :)