Howard W. French, a professor at the Columbia Graduate School of Journalism, has spent years reporting about the massive migration of Chinese people to Africa. Alfred A. Knopf has just published the results of his work: China’s Second Continent: How a Million Migrants are Building a New Empire in China. French, who has served as the bureau chief in Japan, China, West and Central Africa, and the Caribbean for The New York Times, spoke with Columbia’s Weatherhead East Asian Institute about his new book.