I swear I've found this one again before, but about 10-15 years ago, I got a free EBook on the Kindle platform (among many others), and one of the stories I read had a female protagonist in a medieval fantasy world where cruel elves rule the world due to their powerful magic, and have enslaved humanity. The protagonist is in service to an elven lord, and assumes she will be treated cruelly, but he's actually very nice to her. As it turns out, he's a researcher who has found historical evidence that elven magic used to be stronger, and that was because they worked with humans. He's recognized a compatible power in the protagonist, and in the course of working together to unlock this powerful magic, they fall in love with each other. I want to say that there was a plot point that the slave happened to resemble the elven lord's deceased wife with some implication that elves can be reborn as humans.
Unfortunately, the rest of the details have largely forsaken me, including the appearance of the characters (I remember both main characters being fair-skinned, but that could be some sort of racial bias on my part) and that I think the initial magical experiments have deleterious effects on the girl's health, something she initially tries to hide because she still thinks that the elf is only interested in her use in his studies and that, if he finds out she's not 100%, he'll just scrap her (as it is, I think she's relatively uniquely suited to the situation, maybe having a particular magical signature) rather than to let her heal, while we get some internal dialogue from the elven lord about him falling in love with her and being unaware of her discomfort.