The Shadow of the Firefly by Jeff Shannon

The story is about a ten year old girl named Maggie Schoop. She grows up in a stable and safe home in Pittsburgh, Pennsylvania in the 1960's. Her neighborhood, Sycamore Valley, is close to an abandoned asylum. One night while playing baseball with her friends, Maggie thinks she sees a person in one of the buildings. She has no idea that this person has been watching her. The mysterious person and his secrets steadily creep closer to the center of Maggie's life until the shadow engulfs her.