One Ghost Too Many by Drew Stevenson, 1991.

Clark Lannigan meets Sarah Capshaw by accident when she comes to spend the summer with her grandparents in Wilsonburg. Sarah’s grandfather, Conrad Capshaw, is a lawyer, and he is handling the estate of Rodney Maplewood. Rodney Maplewood used to be a museum curator and had an impressive collection of antiques which he kept in his large house, Maplewood Manor.

Maplewood Manor is the oldest and the spookiest house in town. It was once owned by the eccentric Antonia Whitemarsh, who was a spiritualist and believed that her father’s spirit still inhabited the house. Now, people in town think that Rodney Maplewood’s ghost is also haunting the house because lights have been seen in the house at night. Conrad Chapshaw also says that, although nothing is missing, some objects in the house have been moved around.

Sarah persuades Clark and his friend “Frog” Fenniman to join her investigation of the house, but besides the resident ghost, they will also have to deal with a local group interested in psychic phenomena and a mysterious stranger who is paying the local bully to spy on the house.

This is the first book in the Sarah Capshaw Mysteries series.  Sarah and her parents move to Wilsonburg, and she has other adventures with Clark and Frog, all narrated by Clark.

The book is currently available online through Internet Archive.