I love his short stories, going back to his debut, "Born of Man and Woman." The one story he co-wrote with his son Richard Christian Matheson, "Where There's a Will" is a classic I re-read several times over the years.
Anyone who only knows "I Am Legend" as a CGI-heavy Will Smith movie should look up Matheson's terrific story (as a novella, it's sort of sold as a book with some additional Matheson short stories) nobody ever quite did justice to on screen.
Spielberg's "Duel" was a terrific TV adaptation of Matheson's short story, yet it's still probably better on the written page.
I hesitate to even send folks to links that describe those stories. The best way to enjoy Matheson's work for me was to not exactly know what I was getting into when I read it.
I donated and tossed a bunch of books back in 2011 when I was having painting done at home. I just could not part with any books that had Matheson stories in it.
And of course he had his hand in nearly all my favorite episodes of The Twilight Zone.
And of course, he'll be missed...