Lost Boy Lost Girl - Peter Straub

A woman commits suicide for no apparent reason. A week later, her son- fifteen-year-old Mark Underhill-vanishes. His 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 his mother'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 attic to basement with the echoes of a long-hidden true-life horror story, and Tim Underhill 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.