I don't recall the tile or author. I purchased the book via Scholastic books around 1985 in Toronto, Canada (from school).
Cover: A blond girl (I don't remember what she was wearing)
Synopsis: One summer, the blond girl visits her cousin who lives in another city (their mothers are sisters). The Christmas before, the blond girl visited her cousin. Her cousin had a boyfriend but she was sure another girl was trying to steal him away. So the blond girl and cousin want to get back at her. They write her secret admirer love letters asking her to meet somewhere. The "boyfriend stealer" drives out that night but dies in a car crash. The blond girl and cousin are distraught with guilt. That summer, the cousin tells the blond girl that she thinks the boyfriend stealer is haunting her. Strange things have been happening lately. Anyway, we find out that the strange events were due to the new gardener who loved the boyfriend stealer. He knew about the letter and blamed them for her death. But the boyfriend stealer thought the secret admirer letters were coming from the gardener, who would stalk her, so she ignored them. The night of the crash she was actually on her way to visit her own boyfriend. She never intended to steal the blond girl's cousin's boyfriend. They were only friends. Phewph! Very long, I'm sorry, but I'm hoping you could help me find this book.