Some romance novel
So this romance novel that I read when I was way too young for such things. I think it took place in France, or potentially the heroine was pretending to be French or something. I can't remember what time period specifically, but it was a "period" (i.e. not modern) setting. The heroine was undercover spying on some dude who may have been rich and powerful, and her schtick was that no matter how many times they had sex or what he did to her she never orgasmed, so finally he brings in his identical cousin in to help, and of course they all have fabulous messy sex, and THAT'S ALL I REMEMBER. What on earth book was this?