The Hidden Staircase (1930's edition)

Nancy is introduced to the Turner sisters, Pat and Rosemary by Abigail Rem, a character who aids Nancy in the previous, and first volume of the series. They believe that their home is haunted. Nancy's father is being harassed by a crooked character, Nathan Gombet, who threatens violence. Carson Drew, undaunted, agrees to allow Nancy to investigate the house and sends his revolver with her. He will travel to Chicago during the first part of Nancy's stay at the old estate and join her later.
Odd things have been happening at The Mansion. A valuable necklace has been stolen, and canaries get into the house, seemingly of their own accord. Nancy finds it difficult to focus on the mystery at hand when her father fails to arrive home from Chicago in a timely manner - he appears to have vanished. Meanwhile, Nancy discovers that the nefarious Gombet owns the adjacent estate, nearly identical in appearance to The Mansion, nevertheless in a much more dilapidated state. She decides to investigate Gombet's house one rainy night, armed with revolver and flashlight. While roaming around the odd house, Nancy accidentally falls through a secret panel and down a staircase into a tunnel, which leads back to the house in which she is staying.
Nancy reveals her discovery to the Turners in the morning, then they investigate together. The police, acting on Nancy's suspicions, raid Gombet's house, where Carson Drew is discovered to have been a prisoner for days. Gombet is found to have been behind the hauntings as well.