Joanna Brooks's ancestors were among the earliest waves of emigrants to leave England for North America. They lived hardscrabble lives for generations, eking out subsistence in one place after another as they moved forever westward in search of a new life. Why, Brooks wondered, did her people and countless other poor English subjects abandon their homeland to settle for such unremitting hardship? The question leads her on a journey into a largely obscured dimension of American history.

With her family's background as a point of departure, Brooks brings to light the harsh...