Charles Portis, the reclusive author of True Grit, who many considered one of the greatest American novelists, died on Monday in Arkansas at 86, the Arkansas Democrat-Gazette reports.

The cause of death was complications from Alzheimer’s disease, which the writer had been living with for eight years.

Portis was best known for True Grit, which formed the basis for two iconic films. The first, in 1969, starred John Wayne as U.S. Marshal Rooster Cogburn; the second, in 2010, featured Jeff Bridges in the role.

His other novels included Norwood, The Dog of the South, Masters of Atlantis, and Gringos.

“He had this great amount of success with True Grit. I think it didn’t sit well with him,” his brother, Jonathan Portis, told the Democrat-Gazette. “He didn’t like to attract attention. He was comfortable around his friends, but shy around strangers.”

On Twitter, admirers of Portis paid tribute to the novelist.

“So saddened by the passing of my favorite novelist Charles Portis today,” wrote author and rock musician Elizabeth Nelson. “Every one of his books is an endlessly renewable resource of inventive comedy and profound insight.”

Journalist Kaleb Horton tweeted, “RIP Charles Portis, the funniest novelist since Mark Twain. Go get all of his books and somehow get the Coens to adapt all of the ones that aren’t True Grit.”

And U.S. Sen. Tom Cotton of Arkansas wrote, “RIP Charles Portis, an Arkansas original. He enlisted in the Marines served in Korea before a journalism career, which he quit to turn his talent for story-telling to fiction. True Grit is an American classic. He’s been called our ‘least-known great writer.’ Let’s change that.”

Michael Schaub is an Austin, Texas–based journalist and regular contributor to NPR.