Nell Goddin 
The Prisoner of Castillac [EPUB ebook] 

What if Valerie is still alive?
Virtually no one in the village of Castillac holds out any hope. It has been seven years, after all. Seven years since the popular, mischievous Valerie Boutillier disappeared without a trace. The exertions of the gendarmes, led by Chief Dufort, yielded almost no clues, and his failure to solve the case had been one of the main reasons he quit the force.
But then a shred of evidence appears. It might be nothing. But Molly Sutton isn’t afraid to grab hold of it and take it as far as she can.
Will a shred be enough?
And if Valerie is still alive, how will Molly and Ben ever find her in time?