As to the Cthulhu Mythos, I believe I added as much to it as I borrowed. Tsathoggua and the Book of Eibon were my own inventions, and were promptly utilized by Lovecraft. In turn, I borrowed the Necronomicon, which, in its original Arabic, was featured in my yarn The Return of the Sorcerer. Also, I used a quotation from the Necronomicon for a heading on another tale, The Nameless Offspring. Tsathoggua made his debut in The Tale of Satampra Zeiros, and also played some part in The Door to Saturn and The Seven Geases. The Coming of the White Worm was supposedly a chapter from the Book of Eibon; and the book was also featured in Ubbo-Sathla and in The holiness of Azederac (which latter story mentioned two Lovecraftian deities under the slightly altered names of Iog-Sotot and Kthulhut). Eibon himself entered prominently into The Door to Saturn. All of these underlined [italicized] titles, then, can be considered as having more or less relationship to the Cthulhu Mythos.
Dated July 21, 1953