Mystery of the Secret Message by Elizabeth Honness, 1961.

Penny lives with her aunt and uncle because her mother is dead and her father travels on business much of the time, dealing in Asian art.  Thanks to his travels, Penny and her aunt and uncle have quite a collection of Asian art themselves.  However, Penny has just been told that her father’s airplane crashed in the Pacific Ocean.  No survivors have been found, although Penny still has hope that perhaps her father survived and might yet be found.

At the same time, Penny and her aunt and uncle are moving into a house from the apartment where they used to live. Penny is happy about the move because she knows that she won’t have to worry as much about being quiet and not disturbing the neighbors, like she had to do in their apartment. This means that Penny can bring her friends over to the house to play and have parties. Also, their new house has a very special feature: its own private elevator.

Penny loves the new house and soon begins building a tree house with the help of Pete, a boy who lives nearby.  She tells Pete about her father and her hopes that he might still be alive.

However, events take a disturbing turn when Penny receives a package from Japan containing a beautiful wall scroll. The package appears to have been sent by her father, who meant it as a present for her new room in her new house. Was the package sent before his death, or did he somehow survive the crash?

There is also something odd about Penny’s new neighbors.  Penny’s new house is actually half of a duplex, and the new neighbors, the Carruthers, have also recently moved in after renting the other half. When Penny accidentally gets stuck in the elevator and hears voices coming through the wall, she starts to suspect that her neighbors might not be what they seem to be.  They show an unusual interest in her family’s collection of Asian art, asking to see pieces and borrow pieces for an exhibition that Mr. Carruthers is holding at his gallery.  One of Penny’s friends even catches Mrs. Carruthers sneaking around, looking at things uninvited.

When Penny and her friends have a sleepover on an evening when her aunt and uncle are out, someone sneaks into the house, leaving muddy footprints on the floor.  Penny isn’t sure that her aunt and uncle will believe her because they seem to like the Carruthers, so at Pete’s suggestion, she continues to spy on them, using the elevator to listen in on their conversations through the walls.

When her uncle catches her one day, using the elevator without permission (something she is not supposed to do), she finally explains her suspicions and what’s she’s heard the Carruthers say.  Together, Penny and her uncle discover a hidden secret about the wall scroll Penny recently received, which points to a number of secrets that Penny’s father kept from her and the rest of his family for years.  A stranger from the government helps Penny to fill in some of the blanks, but he has a favor to ask in return that requires Penny to take a big risk.