August Derleth, (1909 – 1971) American writer of historical fiction, science fiction and detective stories, and Ascended Fanboy of HP Lovecraft as well as Arthur Conan Doyle. Derleth is mainly credited with founding Arkham House, which published many of Lovecraft's stories in book form, (something Lovecraft himself had dreamed of, although it never really succeed in his lifetime) and popularizing the Cthulhu Mythos.
But Derleth is also a very controversial figure when it comes to his handling of Lovecraft's works and literary memory. Many Lovecraft fans acknowledges that he probably saved Lovecraft's universe and the Cthulhu Mythos from fading into obscurity. But just as many have accused him of Running the Asylum following the years of Lovecraft's death, and claims that he at best misunderstood the themes in Lovecraft's stories, and at worst that he has perverted them and damaged the general public's understanding of them.
Most of this conflict probably stems from the great difference in the two men's faith. Where Lovecraft was an atheist and quite the Nietzsche Wannabe, Derleth was a very religious Christian. Where Lovecraft's universe was indifferent, unsafe and directly hostile at times and, above everything, beyond human comprehension, humans were special and deeply involved in the eternal struggle between good and evil in Derleth's.
Another very controversial and contested change Derleth promoted to the Cthulhu Mythos, was a system where each of the Elder Gods were attached to an elemental force. This led to some debate, as for example, Cthulhu, who in Derleth's system was associated with water, when water, in Call of Cthulhu, was described as the one of the only things that could weaken his powers and force him into hibernation. Other have pointed out that the categorization has made sense, conisdering Cthulhu's octopoid appearnce, and his aquatic servitors. Also, The Dunwhich Horror mentions that Cthulhu is trapped because of being sealed inside his tower, not because of water itself. More troublesome is why Azathoth would be considered an Earth elemental.
For examples of the specific tropes used in his Lovecraftian stories, see the article for his novel The Trail of Cthulhu.
Derleth also created the Solar Pons stories based around a Captain Ersatz of Sherlock Holmes. Some of them make reference to Derleth's Lovecraftian writings. Along with these, Derleth also is known for his Sac Prairie Saga, a series of fiction, historical fiction, poetry, and non-fiction naturalist works designed to memorialize life in the Wisconsin he knew.