The 37-year-old insurance salesman said he had no expectations of writing a bestseller. He just wanted to publish a book after years of dreaming about it — “to get that monkey off my back,” Tom Clancy would tell an interviewer years later.

But when “The Hunt for Red October” was released 40 years ago this month — plunging readers aboard a Soviet nuclear-missile submarine whose captain was seeking to defect to the United States — Clancy would take his first step toward building one of America’s most unlikely publishing empires.