I have not read this book in question, but I have read The Comfort Women: Sexual Violence and Postcolonial Memory in Korea and Japan, by Sarah Soh and based on facts undoubtedly there were women that voluntarily became prostitutes for the Japanese military. Arguably most were forced into prostitution by Korean brokers who acquired girls sold off by their families or misled women into thinking they were doing other work. This same system was in place even after the Japanese military left and the US military entered South Korea:
A local court on Wednesday acquitted a South Korean scholar of defaming women who were sexually enslaved by Japan during World War II through her controversial book.
The Seoul Eastern District Court found Park Yu-ha, a professor at Seoul’s Sejong University, not guilty of the charges, saying academic freedom is a basic right guaranteed by the Constitution.
Park was indicted in November 2015 over her book, “Comfort Women of the Empire,” which has been accused by victims and some civic groups of disputing the coerciveness of the “comfort women” system.
Prosecutors said Park defamed victims by describing some of them as “voluntary prostitutes” or “comrades” of Japanese soldiers.
“The opinion rolled out in the defendant’s book can raise criticism, objection and could also be abused by those who deny the coerciveness of the comfort women system. But it is, in any case, a matter of value judgment that goes over the authority or ability that can be executed by the court under the procedures of criminal cases,” the court said. [Yonhap]
You can read much more at the link.