Gravedigger

From Amazon.com:

A runaway girl leads Dave Brandstetter to confront a California sex cult

Serenity ran away from home when her father was convicted of bribery. For two years, she drifted around the American Southwest, finally finding refuge in the arms of Azrael, a charismatic cult leader whose religion was founded upon blood. Long after Serenity’s disappearance, the police find a mass grave containing six dead girls on Azrael’s property. Thinking his daughter has been murdered, Serenity’s father claims her life insurance, and promptly disappears. Now it’s Dave Brandstetter’s problem.

An insurance investigator with a keen eye and a skeptical mind, Dave is no stranger to savagery. But his trip to the high-priced suburbs of Los Angeles will teach him something new about the depths of human cruelty—and Azrael’s mass grave is only the beginning.

Gravedigger is book six in the Dave Brandstetter Mystery series, which also includes Troublemaker and The Man Everybody Was Afraid Of.