The second book in Tana French's series about the homicide detectives in Dublin. (Ireland, not New Hampshire.) This one concentrates on the female half of the investigatory duo on book number one, Cassie Maddox. The psychic toll she endured in that book was bad enough, but now she's back for more!
And hold on, because this sounds like the least likely premise for a mystery. It's well above average on the Contrive-O-Meter. Before she worked on the murder squad, Cassie worked undercover. Her final undercover persona was "Lexie Madison", terminated when "Lexie" got stabbed by a strung out dealer.
Then came her stint with the Murder Squad. Which was unpleasant enough to get her to retreat to the Domestic Violence department, also unpleasant but boring, (fortunately) not challenging her sanity.
But she's called out to a murder scene anyway, because the victim looks just like her. And (worse) the victim has appropriated Lexie's identity.
So (naturally) the obvious investigational strategy is implemented: "Lexie's" death is covered up; Cassie assumes Lexie's identity once more; and ingratiates herself into Lexie's (weird and dysfunctional) milieu, four friends, graduate students in English at Trinity College. Who all live in a ancient mansion bequeathed to one of them.
So it's kind of gothic and suspenseful. And how likely is it that Cassie won't eventually be found out as a fake? Given that the dead girl was also a fake? Hm.
French is a good enough writer to keep things at least seemingly plausible. Also maintains the general theme that police work can have deleterious effects on your psychological health.