Her Royal Highness (The Royals #4)
C.R. Riley

Even as lines blurred when forced to handle the King's private affairs, I kept my oath.
And when the object of my unattainable affection leaves her husband, no longer technically my responsibility, I found myself darkening her door. In a single breath, I allowed passion to overtake us while knowing it could never happen again. The duty I’d sworn to my King and my country could not be ignored, not when I knew the price we would be forced to pay should he ever learn of our affair.
A sudden threat is brought to life, one she has been keeping secret, lurking over her like a shadow, and places her in grave danger. I am the only person I trust to keep her safe. Protecting her while loving her goes against all the rules, but not breaking them means I’ll lose more than just my job. I'll lose her, the love of my life.
I can't risk that. It's time to put it all on the line and save her before it's too late. Even if it costs me everything.