Pearl of Magic by Emily Deady

Life in captivity is horrible.
But pretending to be a prisoner is worse.
Aizel is a Majis and, against her will, a spy. She is supposed to be uncovering secrets for a cruel king, but spends most of her time listening to the haughty, self-absorbed musings of her captor, Prince Erich.
It would be much less frustrating if she could at least complain about it, but the king has silenced her, taking away her voice so that she cannot wield her magic.
If she fails to gather information from Erich, her family will be killed. But if she reports back to the king, her people’s only hope for freedom will be exterminated. Can she find a way to communicate with her captor and convince him of the truth about the Majis?