The Message in the Haunted Mansion — Carolyn Keene

Nancy Finds Mischief, Mystery, and Danger -- All Under One Roof!
Nancy, Bess, and George have joined the Drews' housekeeper, Hannah Gruen, on a trip to San Francisco. They have come at the invitation of Hannah's friend Rose Green, who has asked their help in renovating a charming old Victorian mansion. But there may be other, uninvited guests as well: visitors from the past...spirits who want the place all to themselves.
Nancy suspects that there is another force at work -- greed. According to legend, a fortune in long-lost gold is hidden on the property, and someone is determined to take it. But in a house full of trapdoors and secret tunnels, falling glass and suspicious fires, finding the truth won't be easy. One misstep, and Nancy won't stand a ghost of a chance!