Nearly 30 years ago, I donated a collection of family letters from the World War I period to the New York Public Library. In her acknowledgment letter, the head of the library’s manuscripts department stated the importance of having “records of the lives of ordinary people in extraordinary times.” I was constantly reminded of that admirable turn of phrase as I read Patricia Luce Chapman’s thoroughly charming memoir, “Tea on the Great Wall: An American Girl in War-Torn China.”