Librarian Note: There is more than one author by this name in the Goodreads database.
Working name of US writer Samuel Mason (1924-1974) who began publishing with "Placebo" for Infinity in 1955; he was married 1956-1962 to Katherine Anne MacLean
David Mason started out writing science fiction short stories in the 1950s, but he is best remembered for his excellent sword and sorcery novel Kavin's World (1969). Between that one and his premature death in 1974, he cranked out four more novels, including The Shores of Tomorrow. Sadly, he never recaptured the charm of Kavin's World. Shores is another mediocre production, covering some of the same themes--i.e., a protagonist leading a band of followers in a search for a homeland, parallel worlds, and the supremacy of the patriarchy. Aside from some battle scenes near the end, the writing here never rises above the serviceable.
One of the chief strengths of Kavin's World, I think, was the first-person voice of Kavin himself. Strangely, Mason chose to abandon that approach in favor of third-person narration in his remaining novels, even in the sequel The Return of Kavin. Since he seemed to produce about one novel a year, and he passed away in mid-1974, the reader wonders if he was in the middle of another novel at the time of his death? Sadly, we'll probably never know.