Lots of authors I admire have been prolific in their output, but I have to admit that that output can be variable. If they are writing for a living, then there will be pressure to keep producing books to keep the money coming in, and not all of them will be up to the standards of their best.
On the other hand, there are authors who wrote a few novels and short stories and then vanished off the scene, leaving us wanting more.
One example is James Stoddard, whose High House and False House owed an obvious debt to William Hope Hodgson (including character names) and were great fantasy novels. The only other work he seems to have produced since those books at the end of the 90s is a retelling of Hodgson’s wonderful but challenging The Night Land for those who find the deliberately archaic prose of the original hard going.
Another great fantasy series is Robert Stalman’s Beast trilogy which looked at the human side of lycanthropy long before the current crop of Twilight clones. The author’s early death robbed us of a great talent.
Another talent robbed by an early death as Tom Reamy who left one novel and one short story collection. The stories in San Diego Lightfoot Sue are variable as Reamy was still learning his craft, but some are wonderful, as was Blind Voices. The circus setting with talented youngsters has been done before and since and for me the pre eminent example is Charles Finney’s Circus of Dr Lao. Finney produced two other short novels and a short story collection – all brilliantly bizarre and eccentric and worth a look if you can find copies.
My final suggestion is Jody Scott who wrote the great Passing for Human and the pretty good I Vampire which are scathing feminist satires including shape changing aliens.