Horror has perhaps the most interesting relationship with literature of all the forms of genre fiction.
One one hand, horror consistently has the most dispersion heaped upon it of any form of genre fiction. Some it of it is deserved, as horror authors have had a long history of indulging in pulp fiction--anathema to the literary elite--since the days of Varney the Vampire and the "penny dreadfuls," which were cheap, pulp novels that cost only a penny.
On the other hand, many of the most oft-studied works of literature fall into the horror genre. In fact, you cannot study Romance-era literature without the horror novel--Dracula, Dr. Jekyll and Mr. Hyde, Frankenstein, etc. Other eras tend to tell similar stories, as tales of terror tend to live on regardless of period.
In the prevailing view, however, horror fiction never has made it out of the murky depths of its pulp roots, despite the unquestionably-literary works that bear its name. Whether it is because of its base subject matter or often "gross-out" style of description, many find it hard to take horror seriously.