The Japanese military "recreation" facilities employed the use of comfort women. Originally comfort women were recruited from Japan, but due to a variety of factors-including the reluctance of Japanese women to participate, the rapidly rising demand, and increasing government pressure-recruiters turned to darker methods, ranging from essentially lying to girls telling them of "guaranteed employment" to just picking up women and forcibly enlisting them into the service. This was especially true in Korea and China, where many women were forced to work in pretty horrible conditions as essentially sex slaves for Japanese troops.

The reasoning for the existence of comfort women ironically boiled down to a public relations concern. There was substantial concern that lustful Japanese servicemen, deprived of female company, would start raping civilians in occupied territories, thus creating a public relations nightmare for a military that already had to deal with foreign criticism of atrocities. Thus, the idea that they could create a "legal" alternative where men could vent their sexual urges seemed to be a good idea.

After the end of the war, during the Japanese occupation, the Japanese authorities for a time managed the Recreation and Amusement Association, which was a similar institution (recruited mostly from poorer Japanese girls who often didn't know what they were getting themselves into) except for the Allied servicemen. It lasted four months before it was disbanded due to moral concerns, although reported rapes sharply increased afterwards.

Yes, it's been common knowledge in the world now for over twenty years when the former comfort women first began speaking out. There's been lawsuits against the Japanese government, international conferences, oral histories, and dozens and dozens of books and articles written about this. So Zamperini's book is rather late to the party so to speak.