It's commonplace nowadays to look askance at the writings of H.P. Lovecraft's correspondent and admirer August Derleth, viewing them as amateurish examples of hackwork pastichery. This is particularly true since the advent of serious Lovecraft scholarship in the last 30 years, which has enabled many people to read HPL without all the posthumous accretions many had previously accepted as the genuine article. Much like Robert E. Howard, Lovecraft's popularity was often helped at the expense of watering down and otherwise bastardizing his singular vision, resulting in many self-professed Lovecraft fans actually misunderstanding what the object of their devotions wrote and believed.
Much of that watering down is the result of August Derleth's tireless attempts to market the Old Gent's works to a wider audience throughout the 1940s and 50s. Derleth rationalized and regularized Lovecraft's intentionally incomplete myth cycle, filling in every empty space and elaborating on each and every reference in order to bring about a consistency that the originals lacked. In addition, he engaged in posthumous "collaborations" with HPL, in which he'd take an idea or some names or even a snippet of text and use it as the germ for a story entirely of his own devising but that, because of the use of Lovecraft's name in association with it, acquired an authority it otherwise would have lacked. In short, it is Derleth, not Lovecraft himself, who is the true father of what we now call the "Cthulhu Mythos," for Grandpa Theobald had no such conception in his mind as he wrote his stories, each of which was a unique creation that might or might not have some connection to what he'd written previously. That is, Lovecraft simply wrote stories, whereas Derleth (and those influenced by him) wrote stories about Lovecraft stories.
1962's The Trail of Cthulhu, published by Derleth's own Arkham House press, is a good example of the distinction. Collecting together six short stories written by Derleth between 1944 and 1952, they tell the story of the thoroughly un-Lovecraftian scholar-adventurer Laban Shrewsbury's battle against the forces of the Mythos on Earth and in realms beyond. Shrewsbury has spent 20 years traveling the cosmos, amassing eldritch lore and arcane power with which to fight his private war against the darkness, in the process becoming nothing less than an occult superhero. It's fun, enjoyably written pulp goodness, but it's about as far from the existential horror of its supposed inspirations as I can imagine.
The fascinating thing is that, as a source for gaming ideas, Derleth's stories are often more rich than are Lovecraft's own. That's not to say that they're better -- that's an entirely subjective judgment I won't make -- but I would certainly argue that the Call of Cthulhu RPG would largely have been impossible without the example provided by Derleth's Shrewsbury and other similar characters. I can and do appreciate the point of view of Lovecraftian purists who think Derleth's enthusiasms did a disservice to HPL's legacy by diluting it with ideas and concepts utterly alien to it. I am myself a dyed-in-the-wool traditionalist and abhor the way that others often strip mine their predecessors' works for things they can use for their own efforts. At the same time, I also concede that popularization can occasionally have a salutary effect on source material, making it not only more accessible but also more generally useful. To some extent, I think Derleth's "Lovecraftian" efforts proved more beneficial than harmful, at least as far as gaming goes, but I am open to being convinced otherwise. At the very least, I think it's quite fair to say that gaming owes a lot to Derleth's ideas, perhaps even moreso than it does to Lovecraft directly.