The Burning Chambers by Kate Mosse

This story is set in 16th century France during the religious wars. Minou Joubert receives a cryptic note saying only "she knows you're alive" and also overhears part of a conversation between her father and a friend hinting at a secret long kept. Shortly after, she and her brother are sent to live with their aunt, where she meets Piet, a rebel fighting for the Huguenot cause and falls in love. Dangers are everywhere and it's hard to tell friend from enemy.
This is a time period I don't know much about, so I had a hard time with the beginning of the book, but once the action picked up it got really good and i couldn't put it down. I look forward to the next book in this series!