Thin Air — Bonnie Lewis

The Haunted Black Forest is full of secrets. Life is busy for fifteen year old Jessica Hartfield. She has a new horse to look after and retrain, and Treasure, her beloved mare, is expecting a foal at any time. Her best friend, Kristin, has been invited to take part in a working student program in Germany. Jessica knows she is going to miss her friend, badly, but there is no way she would ever stand in the way of Kristin's dreams. Then Kristin disappears and it's up to Jessica to try to find her-- before it's too late! Is a tragic ghost story repeating itself?