Tom Clancy, the best-selling spy fiction author behind 28 popular novels like “The Hunt for Red October,” “The Sum of All Fears” and “Red Storm Rising,” died Tuesday night in a hospital in Baltimore, his publisher said. He was 66.

Publishers Weekly confirmed the news in a tweet on Wednesday, although the cause of death has not yet been specified, nor had there been any reports that Clancy was suffering from an illness.

Although Clancy carved a niche for himself in the spy fiction genre with his meticulously detailed espionage and military science thrillers, he never personally served in the armed forces, a fact that continually impressed his devoted following. According to a New York Times article from 1987, Clancy’s preternatural understanding of military operations, though it was based only on reading unclassified documents and interviews with members of the armed forces, was so deep that it was at one point a subject of concern for government officials.

John F. Lehman Jr., then secretary of the Navy, who had lunch with Clancy at the White House in 1985, recalled asking the author who had “cleared” the information that formed the basis of his 1984 novel “The Hunt for Red October.”

''If you were a naval officer, I would have you court-martialed because of all the classified information in your book,” Lehman said he joked to Clancy at the time. However, he said that Clancy had actually “pieced it all together by voraciously reading the open literature for 15 years, things like the Proceedings of the United States Naval Institute.”

According to the article, Clancy first began consuming literature related to the military in the fifth grade and never stopped, eventually moving on to specialized journals and reference manuals that dealt with engineering. Clancy himself addressed the controversy in a 2003 interview with CNN, saying that he was always first and foremost cautious about revealing sensitive information in his books.

"I'll never decide for commercial reasons to put something in that endangers our national security. You just can't do that," Clancy said. "There was one thing, I discussed with a friend of mine in the Royal Navy. I told him a story I knew, and he said, well, Tom, you may never repeat that, as long as you live. And I haven't."

Clancy’s latest novel, “Command Authority” is currently slated for publication on Dec. 3.