Think Twice by Harlan Coben

 Myron Boliter is shocked when law enforcement shows up at his door demanding to know where Greg Downing is. He's surprised because he had given the eulogy at Greg's funeral three years earlier. But the cops are saying that DNA evidence from Greg was found at a recent murder scene. Myron and  his best friend Win start looking into Greg's whereabouts, which opens a dangerous can of worms that could cost Myron his life.

It's a nail-biter of a tale.