In 1942, the Nazis decided that forced labourers in concentration camps would work harder if they were promised sex - so they made female prisoners work in brothels for them.
The brothels form the subject of Das KZ Bordell (The Concentration Camp Brothel) by Robert Sommer, a book that has been hailed as the first comprehensive account of a little known chapter of Nazi oppression in World War II.
Mr Sommer's 460-page work, due to be presented at the Berlin State Parliament tomorrow, explores the origins, structure and impact of the Sonderbauten (special buildings) run by Heinrich Himmler's SS in Germany and Nazi-occupied Europe.
"In the collective memory and written history of World War II, the camp brothels were for a long time taboo," the 35-year-old Berliner told Reuters. "The former prisoners didn't want to talk about it: it was a difficult subject to handle.
"It didn't fit so easily into the postwar image of the concentration camps as monuments to suffering."
Beginning with the Austrian camp at Mauthausen in 1942, the SS opened 10 brothels, the biggest of which was in Auschwitz, in modern Poland, where as many as 21 women prisoners once worked. The last opened in early 1945, the year the war ended.
The chapter is separate from the annals of the Holocaust of European Jews. Jewish women were not recruited as prostitutes, and Jewish men were not admitted to the brothels.
Mr Sommer estimates around 200 women inmates in total were forced to work in the brothels - initially offered the prospect of escaping the brutality of the concentration camps.
"They were promised release after half a year if they served in the brothel. But the promises were never honoured," he said. "Later, the SS just selected women they felt were suitable."
"Jews were not allowed in. Neither were Soviet prisoners of war," he added. "Jewish women did not serve as sex workers."
"The idea behind the brothels was to raise productivity by providing forced labourers with added incentive," said Mr Sommer. "Yet from what I found, it didn't work at all. Only a few people were actually in a physical condition to go to them."