John Feinstein, one of the country’s foremost sports writers and the author of numerous bestselling books, died unexpectedly Thursday. He was 69.

Feinstein died of natural causes at his brother’s home in McLean, Virginia. Robert Feinstein said he discovered John’s body.

Feinstein was a full-time reporter for The Washington Post from 1977 to 1991 and a commentator for outlets such as NPR, ESPN and the Golf Channel. He remained with the Post as a contributing columnist, and he also hosted satellite radio programs on SiriusXM.

“He was very passionate about things,” Robert Feinstein said. “People either loved him or hated him — and equally strongly.”

John Feinstein was working until the time of his death. He was in Washington this week to cover the Atlantic 10 Tournament, and he filed a column Wednesday for the Post on Michigan State coach Tom Izzo.

Feinstein was comfortable writing about an array of sports, but he was best-known for his connection to college basketball because of his groundbreaking book, “A Season on the Brink.”

Feinstein took a leave of absence from the Post in 1985 to embed with coach Bob Knight’s Indiana team, and the book highlighted the author’s flair for the dramatic and ability to capture his subjects’ personalities.

Knight’s reputation for having a hot temper was well-established by then, and Feinstein brought it to life. But Feinstein also effectively portrayed the personal relationships Knight had with his players, which could alternate between warm and abusive.

He went on to write more than 40 books, including “A Good Walk Spoiled” (1995), about professional golf, and “A Civil War” (1996), about the Army-Navy football game. After that book’s publication, he worked for many years as a radio commentator for Navy football.

“The Ancient Eight,” about Ivy League football, was published last year. Feinstein also wrote sports novels aimed at younger readers.

Feinstein is survived by his wife, Christine, son Danny and daughters Brigid and Jayne, as well as Robert and his sister, Margaret.