Detective Diane Webb was determined. The Grim Sleeper, an anonymous killer who’d stopped murdering for 13 years, had resurfaced, and she had a plan for tripping him up. It was the fall of 2008, and she happened to show Detective Lauren Rauch, one of 33 officers who work for Webb monitoring the city’s vast population of registered sex offenders, a recent L.A. Weekly news story headlined, “The Grim Sleeper: The most elusive serial killer west of the Mississippi took a 13-year break. Now he’s back, murdering Angelenos, as cops hunt his DNA.” Webb, a hardcore number-cruncher and sex-crimes expert, floated...