Downfall / Terri Blackstock.

Enhanced descriptions from Syndetics:

From the explosive first pages, Terri Blackstock rockets the suspense level to new heights in Downfall, the third book in the bestselling, award-winning Intervention Series. Emily Covington has turned her life around after a drug addiction, but her family still has trouble trusting her. Though Emily committed herself to a year-long treatment program and has been sober for almost a year beyond that, even her mother walks on eggshells around her, fearing she'll relapse. After her behavior during her drug years, Emily realizes she has a lot to prove. When police discover a homemade bomb under Emily's car, and she then learns the wife of one of her friends was murdered that same morning, she knows things are deadly serious. But who wants Emily dead? And why? A conversation she had with two men, an Alfred Hitchcock movie, and a plan for a double murder all conspire for one explosive ride...and Emily is the only one who can identify the killer and save the life of the next potential victim. As she frantically works to solve this ever more complicated puzzle, Emily finds herself playing right into the killer's hands.