This is the 2014 entry in Tana French's series of novels about murder most foul, as investigated by the Dublin police murder squad. As always, a couple of the detectives here were supporting players in a previous French book.
It's about the reopening of a cold murder case, set mainly at a prestigious boarding school for girls. Which is (as is usually the case) right next to an equally prestigious one for boys. One night last year, one of the rich, good-looking boys got his head stove in with a hoe on the schoolgrounds.
What suddenly causes the case to come alive: someone's posted a pic of the vic in the school's "Secret Place", where the girls are encouraged to post things they wouldn't disclose out loud. And added the caption: "I know who killed him."
The cold-case detective, Stephen Moran, brings the picture to one of the original detectives, Antoinette Conway. Stephen's ambitious and sees this as a possible path to promotion. Conway is hard-as-nails and cynical, but agrees. And they're off to the school to see if they can shake anything loose. (And, eventually, they do.)
The narration alternates between first-person Steve chapters focusing on the newly opened investigation, and third-person chapters describing the events leading up to the murder. Suspicion boils down to two groups: four "Mean Girls", and their antagonists, four "Weird Girls". (Holly is one of the weirdos.) We're obviously supposed to root for the underdogs, but… nope, things aren't that simple.