TL/DR: They’re ok.
Every now and again I think about how cool it’d be to run a game set in Zothique. I’ll dig out my Clark Ashton Smith books and read through the stories looking for inspiration. When the bug hit me this time, I figured “surely somebody has written up a story since the Toad God took old C.A. Smith to his bosom”. I stumbled across a novel and two collections of short stories written by Ran Cartwright: The Darkening, Sorceries Gnydron, and Sorceries Zothique. Technically, the first two, being set in Gnydron, predate Zothique, but what’s 850,000 years when we’re peering a billion years into our future.
I didn’t go into these books with high expectations. Smith had a way with words that few authors can capture. Cartwright has some interesting stories in these collections, but none of them capture the black humor you get in a C.A. Smith story. In fact – and I guess I should insert a spoiler alert here – most of the stories just end in a bloody mess. I almost felt like the author couldn’t figure out a way to end the tale, so he just killed off all his characters to wrap things up.
Out of the three books, I think The Darkening was the best. The collections of short stories can be mined for adventure ideas for games like Barbarians of Lemuria or Conan, but beware of a TPK.
As for gaming in Zothique, G.R. Hager has written up guidelines for D20. I’d likely use Barbarians of Lemuria.