"The Book of O"
https://en.wikipedia.org/wiki/Story_of_O
Story of O is a tale of female submission involving a beautiful Parisian fashion photographer named O, who is taught to be constantly available for oral, vaginal, and anal intercourse, offering herself to any male who belongs to the same secret society as her lover. She is regularly stripped, blindfolded, chained, and whipped; her anus is widened by increasingly large plugs; her labium is pierced and her buttocks are branded.
The story begins when O's lover, René, brings her to the château in Roissy, where she is trained to serve the members of an elite club. After this initial training, as a demonstration of their bond and his generosity, René hands O to his elder stepbrother Sir Stephen, a more severe master. René wants O to learn to serve someone whom she does not love, and someone who does not love her. Over the course of this training, O falls in love with Sir Stephen and believes him to be in love with her as well. During the summer, Sir Stephen sends O to an old mansion in Samois solely inhabited by women for advanced training and body modifications related to submission. There she agrees to receive permanent marks of Sir Stephen's ownership, in the form of a brand and a steel tag hanging from a labia piercing.