I'd especially recommend the story "The River of Night's Dreaming," by Karl Edward Wagner. It's apparently included in The Hastur Cycle, so you'll find it there.
And I'll always recommend the works of Thomas Ligotti as excellent literary horror. While not technically a part of the Hastur mythos (as he doesn't use elements directly from Chambers or Bierce), many of his stories deal with very similar themes of madness, exposure to malign literary influence, and the confusion between literary imagination and reality. Of particular note are "Gas Station Carnivals," "Teatro Grottesco," and "The Bungalow House," all of which deal with the horror of experiencing certain creative works, and all of which can be found in the reasonably-priced paperback collection Teatro Grottesco.
