Chang spent two years on research for the book. She found source materials in the US, including diaries, films, and photographs of missionaries, journalists, and military officers who were in Nanjing at the time of the massacre. Additionally, she traveled to Nanjing to interview survivors of the Nanking Massacre and to read Chinese accounts and confessions by Japanese army veterans. Chang did not, however, conduct research in Japan, and this left her vulnerable to criticisms on how she portrayed modern Japan in the context of how it deals with its World War II past.
Chang's research led her to make what one San Francisco Chronicle article called "significant discoveries" on the subject of the Nanking Massacre, in the forms of the diaries of two Westerners who were in Nanjing leading efforts to save lives during the Japanese invasion. One diary was that of John Rabe, a German Nazi Party member who was the leader of the Nanking Safety Zone, a demilitarized zone in Nanjing that Rabe and other Westerners set up to protect Chinese civilians. The other diary belonged to Minnie Vautrin, the American missionary who saved the lives of about 10,000 women and children when she provided them with shelter in Ginling College. The diaries documented the events of the Nanking Massacre from the perspectives of their writers, and provided detailed accounts of atrocities that they saw, as well as information surrounding the circumstances of the Nanking Safety Zone. Chang dubbed Rabe the "Oskar Schindler of Nanking" and Vautrin the "Anne Frank of Nanking". Rabe's diary is over 800 pages, and contains one of the most detailed accounts of the Nanking Massacre. Translated into English, it was published in 1998 by Random House as The Good Man of Nanking: The Diaries of John Rabe. Vautrin's diary recounts her personal experience and feelings on the Nanking Massacre; in it, an entry reads, "There probably is no crime that has not been committed in this city today." It was used as source material by Hua-ling Hu for a biography of Vautrin and her role during the Nanking Massacre, entitled American Goddess at the Rape of Nanking: The Courage of Minnie Vautrin.