The Hidden Staircase

Nancy Drew is alarmed when Nathan Gombet threatens her father. Gombet sold a piece of land for a railroad bridge through Carson Drew and now believes that he was cheated.
Meanwhile, valuable objects are disappearing from rooms in the Turnbull mansion even while the Turnbull sisters, Rosemary and Florette, are at home in their locked house. Having heard about her reputation for solving mysteries, the sisters invite Nancy Drew to stay in the mansion and discover the thief.
In seeking to solve the mysterious happenings in an old stone mansion, Nancy uses her courage and powers of deduction and tackles a situation that would have appalled a far older person.