I have also been searching for a book I read back in grade 6, around 1984. I don’t recall the author, but I think the title had “secret” in it. The cover had the picture of a girl.
It’s about a high school girl (don’t remember her name, but let’s call her City Girl) who lives in the city. Her cousin lives in a small town, far enough away that City Girl stays for weeks when she does visit. The cousin had a boyfriend, but she believed another girl in school was trying to take him away. During Christmas break, City Girl and Cousin write a letter signed “secret admirer” to lure this boyfriend stealer away. They believe this girl went to meet this admirer, but there was a bad winter storm and she dies in a car crash.
The summer after Christmas, City Girl visits her cousin again because Cousin believes the dead girl is haunting her. Both City Girl and Cousin are threatened and almost killed, all along believing it’s the dead girl’s ghost. Turns out it was the gardener who loved this dead girl; he knew of this letter and was taking his revenge. Solved by a Lost Classics Reader: The Secret by Carol Beach York, part of the Windswept Romace series.
Lost Classics of Teen Lit: Name That Book! 