After School by Jurgen von Stuka
In SUMMER SCHOOL, Lucy von Holt arrived at the Vermont Riding School for pony girl training and BDSM encounters after a tumultuous trip across the Atlantic, courtesy of her weirdo boy friend, who was unhappy that she would not sleep with him.
AFTER SCHOOL tells the rest of the story. Lucy is shipped back to Europe on an air freighter along with some exotic animals and ends up in a convent in Bulgaria where the nuns happily go about improving her demeanor and, among other things, her carriage-pulling team abilities. Just when she is getting the hang of it, the nuns transfer her to a hellhole mine, deep in central Asia, run by a dominant woman.
In flashbacks, we learn that Lucy's erotic personal history includes some BDSM adventures with Glenda, a gorgeous, wealthy young woman she met in Garmisch. Glenda, who has everything, provides her with more kinky lesbian experience and the two "bond" well in a secluded, high altitude retreat in a snowstorm.
However, in real time and after all of this training, Lucy ends up in The Greenhouse, a human botanical garden owned by Fabian, the deviant former boyfriend. She and Glenda, plus several other women, including Jean Groff and Bibi Wolf, two detectives hired by Lucy's parents, are kept hanging around as one might raise potted plants, while Fabian entertains his guests by punishing the girls.
The story features male and female domination, lesbian sex, extreme and intense sadism, box and cage confinement, rubber encapsulation, whipping, vaginal and sexual organ whipping, gags, hoods, multi-port irrigation, and pony girl training. There's even some violence.