“There is only one thing left for you to do,” the author warns vaguely in the final paragraph. “You know what it is.”

When the Rev. Dr. Martin Luther King Jr. received this letter, nearly 50 years ago, he quietly informed friends that someone wanted him to kill himself — and he thought he knew who that someone was. Despite its half-baked prose, self-conscious amateurism and other attempts at misdirection, King was certain the letter had come from the F.B.I. Its infamous director, J. Edgar Hoover, made no secret of his desire to see King discredited.