Moonmist (1986) by Stu Galley and Jim Lawrence.

As described on the box:

In Moonmist, you are a famous young American detective. An old friend, Tamara Lynd, has written you a letter (included in the package), asking for your help. And so you have travelled to England to test your detective skills.

As the story begins, you are outside Tresyllian Castle - the old, dark, hauntingly beautiful castle where Tamara now lives. Tamara greets you, and you meet some interesting guests. But your visit soon turns to mystery, as a trail of riddles and clues leads you to a hidden valuable treasure.

But Tamara is worried about a ghost that is tormenting her. What does the ghost want? Is it jealous of her? Does the ghost want the hidden treasure for itself? Or is the ghost a fake - just someone dressing up to frighten Tamara? If so, why?