Walls Within Walls - Maureen Sherry

After their father, a video-game inventor, strikes it rich, the Smithfork kids move from their cozy Brooklyn neighborhood to a swanky apartment on Manhattan's Fifth Avenue.One day Brid, CJ, and Patrick discover an astonishing secret about their apartment. The original owner, the deceased multimillionaire Mr. Post, long ago turned the apartment itself into a giant puzzle containing a mysterious book and hidden panels—a puzzle that, with some luck, courage, and brainpower, will lead to discovering..