In November 1925, during a brief residence in New York, H. P. Lovecraft was asked by his friend and fellow member of the United Amateur Press Association, W. Paul Cook, to write a study of supernatural fiction for Cook's magazine, The Recluse. The 30,000-word essay, written in late 1925 or early 1926 [revised in 1935], was published in the first and only issue (alongside poetry by Clark Ashton Smith and Frank Belknap Long, and fiction by H. Warner Munn and Donald Wandrei), which appeared in the middle of 1927.

Supernatural Horror in Literature is widely regarded as one of the most scholarly and extensive studies of horror fiction and its practitioners ever written. In an attempt to define the horror story, Lovecraft states that "a certain atmosphere of breathless and unexplainable dread of outer, unknown forces must be present", and the essay contains an obvious bias towards what he terms the "literature of cosmic fear" (a thematic conception that formed the central core of much of his own fiction).

From the Afterword of H. P. Lovecraft's Book Of Horror
on H. P. Lovecraft's essay: Supernatural Horror In Literature
By Stephen Jones and Dave Carson, Barnes & Noble Books, 1996

... Of younger Americans, none strikes the note of cosmic horror so well as the California poet, artist and fictionist Clark Ashton Smith, whose bizarre writing, drawings, paintings and stories are the delight of a sensitive few. Mr. Smith has for his background a universe of remote and paralysing fright — jungles of poisonous and iridescent blossoms on the moons of Saturn, evil and grotesque temples in Atlantis, Lemuria, and forgotten elder worlds, and dank morasses of spotted death-fungi in spectral countries beyond earth's rim. His longest and most ambitious poem, The Hashish-Eater, is in pentameter blank verse; and opens up chaotic and incredible vistas of kaleidoscopic nightmare in the spaces between the stars. In sheer daemonic strangeness and fertility of conception, Mr. Smith is perhaps unexcelled by any other writer dead or living. Who else has seen gorgeous, luxuriant, and feverishly distorted visions of infinite sphere and multiple dimensions and lived to tell the tale? His short stories deal powerfully with other galaxies, worlds, and dimensions, as well as with strange regions and aeons on the earth. He tells of primal Hyperboreal and its black amorphous god Tsathoggua; of the lost continent Zothique, and of the fabulous, vampire-curst land of Averoigne in mediaeval France. Some of Mr. Smith's best work can be found in the brochure entitled "The Double Shadow and Other Fantasies" (1933).

Chapter 8, The Weird Tradition in America
Supernatural Horror In Literature
H. P. Lovecraft (1935)