The Winter Ghosts by Kate Mosse

This book is pretty short and you can tell what's going to happen from the beginning. I still thought it was good. I listened to it on audio and got distracted and had to start over, so the beginning is kind of slow. Once Freddie starts to tell the story of the letter, it picks up and becomes more interesting.

Synopsis:

In the years after WWI Freddie mourns his brother who went missing and was presumed dead, during the war. After spending time in a mental institution, he finds himself traveling in France and gets stranded in a remote village. He meets a beautiful girl named Fabrissa, and they bond over the loss of their loved ones. She tells him about the persecution that happened in her village and how many were forced to hide in a cave in the mountains. The next day there is no trace of Fabrissa and nobody knows who she is. What Freddie does find is a mystery that has been buried for hundreds of years.