Dallas McCord ‘Mack’ Reynolds is the most popular SF author you’ve never heard of.
He was a regular in John W. Campbell, Jr‘s stable of Astounding authors (although his left-leaning politics put him at odds with the rest of Campbell’s mostly conservative writers). He wrote the first original novel set the the Star Trek universe, Mission to Horatius – an honour usually erroneously accorded to James Blish‘s Spock Must Die! Although Blish’s book was the first original adult novel, Mission to Horatius, aimed at younger readers, precededit by two years.
To quote the invaluable Encyclopedia of Science Fiction:
Although he was once voted most popular author in a poll run by the Galaxy Science Fiction group of magazines, Reynolds never received the recognition he deserved for the fertility of his distinctive speculative imagination.