Karen Chisholm
It's a Christopher Brookmyre book. Of course I was going to read it.
From the Blurb:
Is the devil merely the name we give the worst in ourselves?
When private investigator Jasmine Sharp is hired to find Tessa Garrion, a young woman who has vanished without trace, it becomes increasingly clear that there are those who want her to stay that way. What begins as a simple search awakens a malevolence that has lain dormant for three decades, putting Jasmine in the crosshairs of those who would stop at nothing to keep their secrets buried.