Promise Me by Harlan Coben
The 2am phone call is from a young, frightened, lost girl who has called the one adult who had promised to help her if she ever got into any trouble: Myron Bolitar. But his help proves not to be enough. He picks her up from a cold street corner and - against his better judgment - is persuaded to drop her at an unknown address. This is a mistake that he lives to regret, for Aimee Biel's final wave from a darkened porch turns out to be the last time anyone will see her...Driven by guilt and the desperation of her family, Myron decides to break his vow of six years and get involved in the search. But his past will not be buried so easily...and Myron must decide once and for all what he will stand up for if he is to have any hope of rescuing both Aimee and himself.