I know what you’re thinking…
“That’s horror, not Sci-Fi!!”
…but is he? As many of his stories deal with the occult (arcane books, demons, etc), many deal with aliens (incl. The Old Ones), science and technology (either in their attempts to disprove/rationalize, or to breach some metaphysical barrier or some-such).
I can’t say I’ve read every single story, but I have read alot of them.
I just think he may be someone that others who maybe had never thought/heard of, may just find out they love it.