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.

Nancy Drew Secret of the Old Clock

Nancy is 18, and is prompted to help the Crowley kin by her affection for a young child who is Crowley's distant relative. The Hoover Sisters (Changed from 1930 "Horner") and Nancy help one of the Hoover sisters obtain singing lessons to develop her skills; in the original version, they sought to improve their hatchery and dress-making skills. This is a more refined and sedate Nancy; Crowley's initial "heirs" are depicted as very undeserving of wealth; and Helen is older (in preparation for her eventual "write-out" after volume 4 of the revised series - no explanation is made in the original series to introduce Bess and George, although two figures illustrated in the same vein as these girls appear in a 1959 illustration at a camp for girls.) The action is increased significantly, and is faster-paced. Greater detail is given to show Nancy described as wearing a variety of fashions in the book, and her encounter with the undeserving Topham sisters now centers around a torn evening dress instead of a broken vase, as in the original story. Racial stereotypes and all minority characters are omitted, and Nancy encounters an additional difficulty with her car--a faulty top switch, resulting in her chance meeting of the Hoovers while caught in a deluge driving the open car. The final scene, reading of the will which disinherits the Tophams, focuses on the delight of awarding deserving Crowley kin, instead of Nancy's desire to harm the snobbish Topham family.