This is a followup novel to Michael Robotham's Good Girl, Bad Girl, which I read earlier this year. Caveat lector: I can't imagine trying to read When She Was Good without reading Good Girl, Bad Girl first. Don't do that.
The narrative alternates between forensic psychologist Cyrus Haven and teen Evie Cormac. Both have serious psychological problems, and come by them honestly: years back, Evie was found cowering in a secret room in a house occupied by a corpse who had been tortured to death. Cyrus, for his part comes from a broken home: specifically, broken by his schizophrenic brother who slaughtered mom, dad, and sis. Yeesh!
But they have their strengths as well: Cyrus has Holmesian powers of observation and deduction; Evie has an uncanny (and useful) ability to detect lies, combined with a feral skill for survival in hostile environments.
This book is devoted to teasing out the truth of Evie's backstory, how she came to be hidden away in the secret room and who was responsible for putting her there. Unfortunately, that's complicated by the evildoers behind the (previously mentioned) torture and death. Those shadowy folks are still in play, which makes Evie's situation perilous. Without spoiling things too much: they have their tendrils everywhere, trying their murderous best to cover up their crimes.
A lot of gritty, unpleasant, sordid goings-on here. It's set in Britain, so there's also a lot of tea.