Poison and Honey (Dark and Otherworldly Book 1) by Kristen Brand

Infiltrate the fae court. Free the captives. Don't fall for the enemy. Leigh Morgan has one mission: to get revenge on the fae monsters who abducted her sister. It's why she's in Otherworld now, posing as a servant in its dark, decadent palace. She's gathering intelligence and working with a small band of allies to free the humans trapped there. Her mission depends on blending in, so when she accidentally attracts the attention of a cunning prince, it risks everything. Now Leigh has to hide her mission from a dangerously perceptive man who sees right through her—and who's far more complicated than the monster she expected. She can't screw this up, or the humans forced into servitude will never go home... and the man she's starting to love will have her executed for her betrayal. Dark and Otherworldly is a fast-paced paranormal fantasy series with forbidden romance, deadly fae politics, and a heroine who refuses to back down.