Can anyone advise me whether I am right in assuming that CAS' work is in the public domain? I should have thought that a writer who died as recently as 1961 would still be covered by copyright; however, all his stories are on the eldritchdark website which strongly suggests that they are public.

If the stories are public, excerpts from them would come in very handy in my online educational-materials business: I could quote a passage, then analyse the rich vocabulary and style.

Even if still covered by copyright protection, short quotes are permissible in educational situations. Longer passages are questionable and often would depend on the context.

No, AFAIK they are not public.