For horror fiction aficionados, the whole 20th-century canon flows from one fount -- H.P. Lovecraft. Thought I had read 97.8% of everything ever written about him, but was tickled to have just come across this delightful article in the ever-surprising web site tvtropes.org:

http://tvtropes.org/pmwiki/pmwiki.php/Creator/HPLovecraft

...possibly the best one-piece summation of the man & his works anywhere.

He is acknowledged as a if not the primary influence by such as Robert Bloch, Ramsey Campbell, of course Steven King, many others. Another summary: http://www.americanheritage.com/content/man-who-can-scare-stephen-king

In August 1990 Brown University, in HPL's beloved hometown of Providence, RI, organized a scholarly conference on him, on the occasion of the 100th anniversary of his birth. I went, and marveled to find also in appreciative attendance such figures as the macabre cartoonist Gahan Wilson.

This event subsequently morphed into an annual convention of scholars, horror writers, and fans called (of course) The NecronomiCon. Was glad to see it revived this year, after some years' hiatus: http://www.dailymail.co.uk/news/article-2397761/H-P-Lovecraft-honored-hometown-Providence-NecronomiCON.html

Bunch together a group of people deliberately chosen for strong religious feelings, and you have a practical guarantee of dark morbidities expressed in crime, perversion, and insanity.
H. P. Lovecraft

He was a very quotable guy! But he died at the age of 46, apparently having said all he had to say.

Lovecraft in the comics:

http://www.yankeeclassic.com/miskatonic/library/stacks/periodicals/comics/lovecraft/comics1.htm

Arthur C. Clarke's early parody of Lovecraft -- "At the Mountains of Murkiness":

http://books.google.com/books?id=sjksbpH3nT0C&pg=PA88&lpg=PA93&ots=YySw-SEIal&focus=viewport&dq=%22slog-wallop%22&output=html_text