Secret Apartment — Natalie Fast

Eleven-year-old Jillian's life is changing and she's not happy about it. First of all, her mother got remarried and they moved from their house in Pennsylvania to her new stepfather's apartment in New York City. Second, her new stepfather has a daughter, which means Jillian now has a stepsister. An evil teenage stepsister who talks on the phone all the time and has a stupid dog. And finally, Jillian is being sent to camp for the summer where she has to endure Peace Circles and sack races.
But Jillian gets a break when Mrs. W in the penthouse apartment asks her to catsit for the summer. Mrs. W's apartment is full of art and art supplies, vintage dresses and hats, and cakes and soda, and most important, it offers a great view of the windows in the building across the street. Watching the neighbors is funâ€"until Jillian and her new friend Emily see something that doesn't look right. And it's up to them to save the day.