Into Thin Air — M.J. Allaire

While spending summer vacation in a small North Carolina town, 14-year-old Bobbie Farnsworth becomes fast friends with a young girl named Sam. As their friendship grows, Bobbie learns Sam's twin sister, Serena, disappeared without a trace three years prior. Although the authorities believe Serena was kidnapped, Sam is convinced something else happened to her sister; she just doesn't know what. As the end of their vacation grows near, Bobbie browses through pictures she's taken during their stay and is shocked by one photograph in particular. Standing atop the strange, rock wall, the undeniable ghostly form of a young, blonde girl stares out across the ocean. Serena, Bobbie whispers, absolutely certain of the girl's identity. Had she really been kidnapped, or had she simply vanished into thin air?