Raven and the Slave

Patty is naked, helpless, her arms bound behind her back, and so very beautiful. She is also highly embarrassed, claiming to have tied herself up and now forced to seek the help of Raven, because she is unable to undo the knots. From that surprising incident, a new relationship forms between the two young women, a relationship based on domination and submission. Soon, Patty is regularly being tied up, spanked, and forced, not at all against her will, into sexual acts by the older girl.
The roles of Mistress and submissive fit them well, and perhaps they would have lived happily ever after, except for a random meeting with another couple of young women who share the same type of relationship. From that point on, things get complicated - and very nasty. Note: originally this book was published as "My Sister, My Slave." But that didn't set well with some distributors so it was changed.
Now they're just friends, not sisters.