Desert wives : a Lena Jones mystery / Betty Webb.

Enhanced descriptions from Syndetics:

Now in its second edition, Betty Webb's Desert Wives is a startling, real look into the polygamous communities of Northern Arizona. When private detective Lena Jones helps thirteen-year-old Rebecca escape from Purity, a polygamy compound hidden in a desolate area straddling the Utah/Arizona border, she uncovers more than she bargained for. Rebecca's mother has now been arrested for the murder of Prophet Solomon Royal, Rebecca's intended husband. So Lena enters Purity masquerading as a polygamist wife to uncover the real murderer. What secrets are the Circle of Elders so desperate to protect?

Lena thinks she's put her own past behind her, but the sins of Purity's mothers and fathers force her to reexamine the scant memories of her early childhood. At the age of four she was found lying unconscious by the side of an Arizona highway, a bullet in her head. Raised in a series of foster homes, Lena does not remember her real name or the names of her parents. Are Lena's past and this new case somehow connected?