"The Ghost of Blackwood Hall" by Carolyn Keene (Nancy Drew Mysteries #25)

When Mrs. Putney seeks Nancy Drew’s help in recovering her stolen jewelry, the search for the thieves takes the teenage detective and her friends Bess and George to the colorful French Quarter of New Orleans. But the quest is hampered by the strange behavior of Mrs. Putney and two young women who are being victimized by so-called spirits. How can Nancy fight these unseen perpetrators of a cruel hoax? And how can she help the gullible victims when the spirits warn them not to have anything to do with Nancy?