Fiction by Harlan Coben.
This was an exciting thriller. Although it's listed as "Myron Bolitar #8," I feel you could read it without having read the previous seven Myron books. (I've only read #1 and #2 myself, and they were not as good as this one, or my favorite one: #11 Home.)
This book focuses on a pair of missing girls who just turned eighteen and had attended the same high school. They may or may not be runaways, and their disappearances may or may not be related.
I also read recently by this author: Just One Look