The Secret of Stonehouse by Lynn Hall, 1968.

Heather has lived her entire life (as far as she can remember) in Scotland with her grandmother and her uncle, Donald.  Donald has raised her since she was small.  He’s been like a father to her, and she loves him like a daughter.  However, he recently decided to move the two of them to the United States, taking them to a small town in Wisconsin.  Heather can’t understand the reason for the move, and for the first time in her life, it seems like Donald is keeping secrets from her.

Donald seems oddly concerned that Heather shouldn’t tell people that she is adopted, something that he’s never seemed concerned about before.  Heather has asked him about her parents before, but all he can tell her is that his wayward brother Ewen brought her to the family farm in Scotland, saying that she was his daughter and that her mother was dead.  Ewen simply left her with Donald, never trying to see her or talk to her again and never sending her any money. Heather also knows that, although says that he’s going out to search for a new job, he’s been hanging out in other places, spending time with the mysterious Mr. Worley.

Heather makes friends with a boy named Gus who lives nearby.  Gus lets her ride one of the horses that his family owns, Cloud, and invites her to go riding with him sometimes and participate in local riding events called “shodeos.”  Heather loves horses and enjoys their rides together.

On one of these rides, the two of them go near a large, old, stone mansion that gives Heather a strange feeling.  Gus’s family tells her the tragic story of the family who used to live there, the Selkirks.  They were wealthy, but young John Selkirk was killed in an accident the day that his beautiful young wife, Molly, gave birth to their only child, a little girl named Hebron.  John’s parents never recovered from the loss of their son and passed away soon after, leaving just Molly and the baby.  However, when Hebron was only three years old, she was apparently abducted for ransom and later murdered, and her mother died soon after.  The story makes Heather uneasy, and the house gives her a strange feeling, like she’s drawn to it.

However, other sinister things start happening.  Someone in a car that looks disturbingly like Donald’s tries to run her and Cloud off a bridge.  When Donald spends the night away from home “on business”, someone sneaks into the house.  Heather begins to realize that someone is out to get her, some mysterious person means her harm.  Memories, dangerous ones, are beginning to surface in Heather’s mind, and someone is determined to try to keep her from remembering.

Part of the mystery is pretty obvious (at least, I thought it was, and you might guess it from my plot description), but the part that I didn’t guess was who was behind it all.