The Hidden Staircase (1930) by Carolyn Keene [my version = 1959, revised text]
Nancy's friend, Helen Corning, asks if Nancy would like to investigate a haunted house and when Nancy says yes (of course!) plans to bring her great-aunt Rosemary Turnbull to give Nancy all the details. Meanwhile, a nasty character by the name of Nathan Gomber arrives to warn Nancy that her father is in danger and she'd better stick close to him. Carson Drew has been acting, with other lawyers, for a railroad company which bought land from various home owners. One of the lawyers did not have paperwork signed correctly and now that home owner has disappeared and the group are holding out for more money. When Nancy tells him about the visit from Gomber and that she's worried about the danger, Carson insists that there's nothing to worry about. He's going to follow a lead to Chicago in hopes of finding Willie Wharton and getting the signature verified properly. There's no reason why Nancy can't investigate the strange goings on at the Turnbull house.
So Nancy and Helen go to Twin Elms, the Turnbull home, where the mysterious incidents continue. Radios get turned on when no one is around, furniture moves mysteriously, an owl appears in a locked room, and jewelry disappears. Nancy becomes convinced that the old house must have a secret passage and the search is on. But before she can wrap up that mystery, her father disappears and she has to find him and solve the mysteries surrounding the railroad deal as well.
The 1959 revised edition is one of a set of tweed-covered Nancy Drew books that my mom passed on to me when I was in second grade. Those six books hooked me on mysteries for life. Staircase was always one of my favorites. It has everything--spooky goings-on, a hidden passage, a definite nasty bad guy, and just enough danger to make things exciting. It was fun revisiting this childhood favorite and I was glad to see that this one holds up very well. ★★★★ and 1/2.
First line: Nancy Drew began peeling off her garden gloves as she ran up the porch steps and into the hall to answer the ringing telephone.
Last lines: "And we can make all the plans we want," Nancy replied with a grin. "There won't be anyone at the listening post!"