On the back cover of his new book, “The Threat: How the F.B.I. Protects America in the Age of Terror and Trump,” Andrew G. McCabe looks preposterously fit (he competes in triathlons). His hands are at his hips, gunslinger style. It’s as if he were a kind High Plains sheriff who had stumbled upon an intolerable rodeo of villainy.

McCabe’s prose is lean, too. (Not that he wrote this book. In his acknowledgments, he thanks “a great writing and editing team.”) The first sentence demands to be read in the voice of Jack Webb from “Dragnet”: “Between the world of chaos and the world of order stands the rule of law.”

McCabe is, of course, the former deputy director of the F.B.I. who was fired last March, just 26 hours before his scheduled retirement. He was briefly the F.B.I.’s acting director, after the dismissal of James B. Comey. The president hooted on Twitter: “Andrew McCabe FIRED, a great day for the hard working men and women of the F.B.I. — A great day for Democracy.”

This lawman, a registered Republican for the entirety of his adult life, may have been driven out of Dodge. But he has dusted off his white hat and returned with a memoir that’s better than any book typed this quickly has a right to be.