A psychological, complex mystery where what's going on in the detectives' psyches is as much a part of the story - maybe more - as solving the central murder.
Cassie is a former undercover cop in Dublin who used to work in Murder but after an earlier case messed with her head, switched to Domestic Violence. When a body is found that looks just like her and has identification with a name she used to use when she worked undercover, her former boss from undercover (Frank) urges her to go undercover as the dead girl to identify suspects in her murder. Cassie's boyfriend, Sam, still works in Murder and is in charge of the case, and doesn't like the idea, but she agrees to do it. She moves in with Lexie's roommates, who are a pre-Raphaelite brotherhood kind of household: Daniel, who inherited a big house in an Irish village outside Dublin and invited his Trinity College friends to live with him; and Abby, Justin, and Rafe. The household is weird and compelling and dreamily appealing - they cook together, they study together, they play cards and discuss intellectual topics, and they present a united front against the outside world. As the story unfolds, we learn a little more about the Irish village; the students are outsiders, but there's also the history of Daniel's family's relationship with the village. And then there's Lexie, the dead girl: who is she, why did she take Cassie's undercover identity, was there something in her past that led to her murder? The four roommates slowly become more individual; they have a rule not to talk about their pasts, which suits Lexie fine.
Let me get my comparisons to In the Woods out of the way. I found this one more satisfying, because there were fewer loose ends left for me to wonder about at the end of the book. One of the things I had trouble with in In the Woods was also present here, although to a lesser extent: the main character's behavior in a couple of spots was frustratingly irrational. Cassie struggles with divided loyalties and conflicting motivations throughout; she gets sucked into her undercover role and falls in love with Lexie's friends, for one thing, but she's pretty aware of that, and mostly her inner battles over her job versus the undercover role felt real. The thing that was too far for me to go along with her was where her loyalty to Lexie and identification with the dead girl and sense that she's on a mission from a ghost drove her to take irrational risks. But like I said, this was only in a couple of spots.
If you're wondering whether you need to have read In the Woods first - I don't think so, and in fact I think I would have enjoyed this more if I hadn't, because I couldn't remember enough about what happened to Cassie in that one, and if I hadn't read it I wouldn't have spent time wracking my brain every time Operation Vestal was mentioned. I read ITW as an audiobook I got from the library and thanks to all the kind people avoiding spoilers, couldn't find any explanation of exactly what happened and how it messed her up. If I hadn't read it, I'd just accept it - okay, a previous case disturbed her and made her switch from Murder to Domestic Violence, let's move on and read the current story. On the other hand, since I don't remember ITW well enough to know, it's possible it would have added depth and nuance that I totally missed, if I'd read this right after I read the other one. (If anyone has an opinion and would like to mention it in the comments, I'd really appreciate it!)