Awkwardest writing ever. The characters are not believable, their actions, conversations and feelings (the depth of which is expressed through gems of sentences like, "She liked him a lot, but more than that... she _really_ liked him...") are not believable. The whole premise of the book is not believable. These kinds of businesses function pretty well all over the world without the need for "keeping them [sex slaves] held captive" (an actual quote, yes, "keeping them held captive") to the extent described, with these slaves having short "shelf lives" (i. e. being subject to being killed). Which means unnecessarily high risks and unnecessarily high expenses for output which is achievable through business processes with much lower stakes for everyone involved. People are usually not particularly wont to increase their count of criminal charges for no good reason, unless they are a special kind of very dumb psycho-/sociopath. But nope, the author informs us the main villain is smart and tries to show us her human side. NOT BELIEVABLE given the ludicrous premise. A person who genuinely thinks a captive should be grateful for "a life of luxury in his three-by-three room" (the author is NOT being ironic here, he is ACTUALLY trying to be sinister, and this is a good example of the ridiculousness of the whole book) would not be able to not creep out every single human being they would strike up a conversation with and thus would hardly be suitable for a membership in a country club; there is any number of social mores they would have violated multiple times throughout their lives and would be shunned by everyone.
What is described is an operation that has been going on for a long time, and only a particular detective taking a particular interest in a particularly rebellious sex slave finally busts this den. Nope; if someone had actually decided to set up something like this (and was a person of the kind that would be able to set up something like this), things would have gone wrong in a myriad of ways a long-long-long-long-long time ago.
By way of some minor peeves (which are nonetheless extremely irritating because the book is chock-full of them)—the way immigrant characters talk is not believable. It might surprise the author, but Germans actually have the verb "to be" that functions much like its English counterpart (English being a Germanic language), so a German would NOT say "I from Bavaria". Even a Japanese person, after so many years in a foreign country, wouldn't say things like "I not know" and "I sorry", because language gets picked up in whole phrases, so if the person has heard "I don't know" or "I'm sorry" two or three times, they would start repeating it. No one speaks a foreign language by translating content words separately on their own, _especially_ when surrounded by native speakers for many years.
These constant on-the-nose (and ridiculous) reminders that "these are foreigners" only add to the stilted, heavy-handed language throughout. You get a ridiculous detailed description of every single clothing item a person took off before taking a shower; there are multiple instances of the reader being provided random unneeded details about clothing colours ("she grabbed her jacket from her chair, put it over her white blouse, and picked up her black bag"—all of that is really essential information!!!!). You are told the character traits of the characters ("pragmatist", two times half a page apart). You are told what the situation is supposed to be like ("a waking nightmare"). A person who is trained in firearms and spits out details about a certain gun when the time comes, first specifies what she needs in a gun in the following way, "The gun, it has to be small enough to fit in my bag". No, people who know their way around a gun usually have certain preferences and would specify an exact make and model or two.
I persevered until the end because this was actually the worst book I've ever read (so far), and I was in a way awed by how bad one could write and still think their writing was worthy of being made into a book, so decided to finish it and write a review. I'm seriously shocked by the ratings of this book here on Goodreads (I'm new... and I guess this is a good lesson in how much one should trust Goodreads reviews).