The Secret of Stonehouse — Lynn Hall

Heather had never known a horse like Cloudy, the silver gray mare. It was wonderful to race through the beautiful Wisconsin countryside on Cloudy's back--and to be friends with Gus Ohlander, Cloudy's young owner.

But there were dark questions in Heather's life, questions she could not answer: Why had her father abandon her so many years ago? Why had her Uncle Donald, who had raised her so tenderly, become mysterious and secretive? And what was the murky secret of the deserted mansion called Stonehouse?

As Heather became more fascinated with Stonehouse, she encountered a series of near-tragic accidents. But no "accident" could explain what happened one midnight, when someone tried to kill her...