In a media landscape so polluted by politicians addicted to cheap thrills (Matt Gaetz, Marjorie Taylor Greene, the Orange Monster) and the pundits addicted to them (Sean Hannity, Laura Ingraham, Steve Bannon), the success of Heather Cox Richardson is much more than a blast of fresh air. It’s a bona fide miracle. The Boston College history professor started writing her newsletter, Letters from an American, almost four years ago. Today her daily dose of common sense about the day’s news, wrapped in an elegant package of American history, has a remarkable 1.2 million subscribers, making her the most popular writer...