If you haven't read Clark Ashton Smith, very quickly: know that Smith was one of the great fantasists of the early 20th-century American pulp era, contemporary with his friends H. P. Lovecraft and Robert E. Howard. Like them Smith wrote weird tales, stories of mystery and horror, often in outlandish locales. Like Howard, some of his stories involve medieval-ish kings, swordsmen, magicians, etc. Like Lovecraft, Smith is fond of extreme vocabulary and arcane texts. Unlike them, Smith preferred a dark sense of humor, where stories end by harsh irony. Narratives end not with shrieking madness (HPL) nor heroic tragedy (Howard), but with twisted, mordant melancholy.
If you have read Smith, Zothique is a dying Earth world, of the sort subsequently developed by Jack Vance, Gene Wolfe, and the Nausicaa anime. It's late in our planet's history, and the world looks different: continents have reassembled into a new form of Gaia, magic sometimes works, and ruins are widespread.
Most of Zothique's stories involve exploration, voluntary or otherwise, which gives Smith the chance to show off his fabulous language and fertile imagination. Protagonists run into demons, cultists, necromancers, and tricksters. But plots don't drive these tales so much as the lush, fervid atmosphere.
On Zothique, the last continent on Earth, the sun no longer shone with the whiteness of its prime, but was dim and tarnished as if with a vapor of blood. New stars without number had declared themselves in the heavens, and the shadows of the infinite had fallen closer. And out of the shadows, the older gods had returned to man: the gods forgotten since Hyperborea, since Mu and Poseidonis, bearing other names but the same attributes. And the elder demons had also returned, battening on the fumes of evil sacrifice, and fostering again the primordial sorceries.Relish these stories, or dive through the lot and check the skies for a low, red sun.