It may never occur to you to read anyone other than HP Lovecraft when reading Cthulhu tales. And it might never occur to you to read Cthulhu at all. But if you do read Cthulhu Clark Ashton Smith is a writer who you will want to add to your list. He was skilled more as a poet and some say as a sculptor than as a long form prose writer, but this is not to say he was at all anything but great as a prose writer. His work was creepy, dark, evocative of horror, and made other writers of Cthulhu tales pale in comparison.
Smith's work was considered by his peers to be superlative. His critics, however, were often unrelenting in their criticism with issues of taste and issues of what should and should not be in a horror tale. Despite the fact that his work was not prurient or lurid, the language use was so vivid and powerful, people suggested that Smith was in love with rotting flesh, and the corrupted corpse. His work was more popular during his lifetime, and pursuing it now is expensive. But the pursuit of it is well worth the cost.