Deputy Cass Bollman was about to enjoy a coffee break at a gas station in Iowa when the alert came across his radio: “Asian male wearing a suit walking through a farm field. … Nature of incident: suspicious.”

Bollman raced to the scene, a little town northeast of Des Moines, where he talked to a farmer who had spotted the odd figure in the field. A few minutes later, Bollman had pulled over an SUV driven by Robert Mo, a Chinese national.

So begins one of the most unexpected stories of espionage ever told — in this case, by Minnesota journalist Mara Hvistendahl in her new book, “The Scientist and the Spy.”

Friction with China

America’s...