Frost (Frosted Fae #1) by ED Lee:
 
The King of the human realm waged war on the fae, taking the young fae Prince back as his prisoner after slaughtering his family in cold blood. For two and a half decades, Prince Crispin has sat in the dungeons of his mortal enemies castle.

Rory is a young commander, desperate to prove herself to the King she serves under. But her blood has a secret that could ruin her life if the King finds out what she really is. She hides under the mortal King's nose as he continues to wipe the earth of all the fae with unrelenting force. Her fate lies within a choice between duty and righteousness.

Sterling has always been right there to challenge and push Rory to her wits end, rivalling for the same attention and favour from King Oberon. His own decisions end him up on guard duty with her, watching over the frosted fae in the cells.

A challenge. A ring. A spell book.
The fate of the fae and the two Kingdoms rests in the hands of these three.