Sorry to hear this. I am glad that the movie gave him more widespread recognition in his final years. His stuff wasn't always my cup of tea, but I admired the fact that a guy like him (a Joe Blow) could make it as a writer.
"I like your Christ, I do not like your Christians. Your Christians are so unlike your Christ."
Sad news. I admit I've only read I Am Legend (or rather, heard it on audio book), but Matheson was prolific. Here's a 2011 story on why Hollywood loved his works as source material for movies. (theatlantic.com)