Lisette Ashton 
The Black Room [EPUB ebook] 

The submissive trainees at the Pentagon Agency derive pleasure from even the most demeaning tasks. Their lives are dedicated to sexual servitude; they enjoy pain and humiliation on a daily basis. There is only one punishment they try to avoid: the black room.
When private investigator Jo Valentine is assigned to infiltrate the Pentagon Agency, she is prepared to do anything to get results – but nothing can prepare her for what lies in the black room. The more she discovers about the agency, the more she learns of her passion for dark and bizarre sexual games that go beyond anything she has experienced before.