The Mercenary by Cecelia Mecca

A fake marriage is the last thing he wants . . .and the one thing he needs.
Guy Lavallais never expected to marry. But when a nun overhears him making treasonous overtures to a bishop, she makes a shocking ultimatum: marry her or she’ll tell all.
And there is plenty to tell.
Guy is a member of the Order of the Broken Blade, a brotherhood of knights seeking to tame King John—or if that fails, to overthrow him.
If the wrong person learns his secret, Guy will die a traitor’s death.
Of course, Lady Sabine isn’t a nun at all.
She was brought to the abbey against her will, and she’ll do anything to escape. Even bribe a wickedly handsome mercenary. She tells herself the marriage doesn’t matter—it’s a temporary measure to protect her from her guardian.
Once she and Guy part ways, she’ll be free to pursue her passion. And yet, the more time she spends with the mercenary, the more he awakens other passions.
Their pretend marriage begins to feel very real, but Guy’s mission for the order soon threatens both their arrangement and their very lives.