The Likeness by Tana French.
This is the sequel to French's debut novel In the Woods, which won both the 2007 Edgar and the 2008 Anthony for Best First Novel.
I thought I'd posted a review of it last August, but apparently I did not, and I apologize.
While In the Woods was written from the point of view of Rob Ryan, The Likeness is in the words Cassie Maddox, who was his partner in the first book.
Cassie has transferred from Murder to Domestic Violence, and is involved in a relationship with Sam O'Neill, whom we also met before. The story begins when Cassie is summoned to a murder scene by Sam. The victim is one Lexie Madison, a character Cassie had created when she was working undercover in order to infiltrate a drug ring.
Cassie's boss from Undercover is also there, and he asks her to reassume the Lexie persona to try to flush out the killer.
French is an amazing writer. She gets readers so deeply into the mind of her protagonist that we almost become the character ourselves. Although the premise of the story is a little farfetched, we never question it, and like Cassie, we want to see the case to its solution.
Here's hoping we see another Dublin novel from Ms. French soon!