“The Killer of Little Shepherds: A True Crime Story and the Birth of Forensic Science,” by Douglas Starr (Knopf, 320 pp.) juxtaposes the stories of Joseph Vacher and Dr. Alexandre Lacassagne. Vacher was a ruthless serial killer in late 19th- century rural France; Lacassagne was the scientist who pioneered criminal forensics – the field that would bring men like Vacher to justice.