Lost Boy Lost Girl by Peter Straub

Nancy Underhill commits suicide for no apparent reason. A week later, her son -- fifteen-year-old Mark -- vanishes. The boy's uncle, novelist Timothy Underhill, searches his hometown of Millhaven for clues that might help unravel this horrible dual mystery. He soon learns that a pedophilic murderer is on the loose in the vicinity, and that shortly before Nancy's suicide, Mark had become obsessed with an abandoned house where he imagined the killer might have taken refuge. No mere empty building, the house whispers from basement to attic with the echoes of a long hidden true-life horror story, and Tim comes to fear that in investigating its unspeakable history, Mark stumbled across its last and greatest secret: a ghostly lost girl who may have coaxed the needy, suggestible boy into her mysterious domain.