Game of Spies by Pamela Mingle

Life as a lady-in-waiting for Mary Queen of Scots isn’t at all what Isabel Tait expected. But she must either tolerate the mindless partying and ceaseless backstabbing of Mary’s other ladies, or risk being sent home to a forced marriage. She’d almost rather return to her family—if not for a certain handsome gentleman who arouses a new desire in her, something she’s never before experienced. Of course, he’ll never notice her…
Gavin Cade is on a mission. Tasked with spying on the Queen, he’s found the perfect woman to help. Sweet, shy Isabel is an easy target for his charms. Before he knows what’s happened, he is the one beguiled. With the fate of England in the balance, he must keep his plan to expose the treasonous queen a secret, or he will never seduce his sweet English rose.
Because there’s one thing he’s learned about Isabel Tait…she’s got a backbone of steel and may never forgive him.