|Clark Ashton Smith (1912)|
courtesy of Wikimedia
The three of them borrowed elements from each other, with Smith creating the toad-god Tsathoggua, Mordiggian the Great Ghoul and the wizard Eibon whose presences in Cthulhu mythos and elsewhere are well-known.
Eldritch Dark is a prime resource for fans of Smith, with hypertext versions of his writing, a Zothique d20 guide and active forum. You can find some stories in audiobook form at Huffduffer (hat-tip to Jim Garrison for that).
There is some contention around copyright for Smith's works especially for his Averoigne, Hyperborea, Poseidonis, Xiccarph and Zothique cycles, though this hasn't stopped some settings drawing inspiration from his tales (e.g. D&D Known World/Mystara). While he's not explicitly referred to in Appendix N, he is surely one of the unnamed inspirations for AD&D.