Promise Me by Harlan Coben

Bachelor Myron Bolitar looks upon his friends children as "sort of" his own and he cares and is concerned for them as well as their parents. When he overhears a teen-aged daughter discussing partying and drinking and driving, he makes two neighborhood girls promise him that if they are ever in a dangerous situation, to call him. He will come get them, no questions asked. When his phone rings at 2 am and he responds, picking up the girl, taking her to her friend's house, his nightmare begins. When she doesn't show up at home, Myron decides to track her down, the investigation leading to the disappearance of another missing girl also. Things being the way they are with young adults today, this story seems to have a ring of truth. A very good read.