The bottom line is murder

From Goodreads: Corporate investigator Paul Marston witnesses the crash of a plane which kills Jack Carlisle. Since Carlisle helped him get started, Marston decides to look into Jack's death. Uncovering evidence of digitalis poisoning, Marston searches among Jack's relatives and business associates for motive and opportunity. He discovers blackmail, family in-fighting, corporate finagling, and mob connections. Paul Marston and Angel Cantini #1