The Good Sinner's Naughty Nun (Conquerors #4)
Georgia Fox

Thierry was hoping for different company, something decidedly less virginal. Now, resolved to a very dull journey, where his only pleasure is arguing with a naughty, quarrelsome nun named Sister Vivienne, he has no warning of the storm ahead.
Vivienne is also on a mission. The priceless vase hidden inside that casket of relics is her pass to freedom. All she has to do is find the key in Bonnenfant's breeches. Apparently he's all brawn and no brains. Seducing him to get that key should be her easiest task ever.
But she's reckoned without this particular sinner's last remaining scruples. And the reckless, bloodthirsty warrior's only fear. Of nuns.