Wellman, Manley Wade
I have no idea who that is, and I've never read any of his works. Gygax does not actually list which of his works influenced him, but I'm pretty sure I know what it is after doing a bit of research.
https://en.wikipedia.org/wiki/Manly_Wad ... kin=vector
https://en.wikipedia.org/wiki/Silver_Jo ... kin=vectorThree of Wellman's most famous recurring protagonists are John, a.k.a. John the Balladeer, a.k.a. "Silver John"
John, whose last name is never revealed, is a wandering singer who carries a guitar strung with strings of pure silver. He is a veteran of the Korean War and served in the U.S. Army as a sharpshooter (in the novel After Dark, he mentions that his highest rank was PFC). In his travels, he frequently encounters creatures and superstitions from the folk tales and superstitions of the mountain people. Though John has no formal education, he is self-taught, highly intelligent and widely read; it is implied that his knowledge of occult and folk legendarium is of PhD level (in the novel The Hanging Stones, he receives word that Flornoy College is awarding him an honorary doctorate for contributions to folklore and folk music). This knowledge has granted him competent use of white magic, which he has used on occasion to overcome enemies or obstacles, but it is primarily his courage, wit and essential goodness that always enables him to triumph over supernatural evils (although the silver strings of his guitar and his possession of a copy of The Long Lost Friend are also powerful tools in fighting evil magic), while basic Army training allows him to physically deal with human foes. He has an implied mystic link of some sort to John the Baptist, and much of his personal philosophy can be traced to a "primitive" Gospel-based Christianity. On one occasion, he is "employed" by the State Department to investigate on their behalf a possible instance of Satanism (see the novel The Lost and Lurking for full details).