”York, shadow cipher” by Laura Ruby

Book One of the York series. Tess and Theo Biedermann and their friend Jaime Cruz live in a Morningstarr apartment-until a real estate developer announces that the city has agreed to sell him the five remaining Morningstarr buildings. Their likely destruction means the end of a dream long held by the people of New York. And if Tess, Theo and Jamie want to save their home, they have to prove that a forgotten puzzle is real. Which means they have to solve it.