OMG, I just read the MURDER OF BINDY MACKENZIE by Jaclyn Moriarty, which isn't even out in America yet. It is SO SO SO GOOD. It comes out October 1.
Moriarty wrote The Year of Secret Assignments and Feeling Sorry for Celia. It helps to have read Secret Assignments when you read Bindy. All three take place at the same high school.
Again, it is so so so good.
Interestingly, when googling Moriarty (she has no website), I learned two of her books have different titles in Australia (which is where she's from). I like the American titles much better. The Australian titles are: Becoming Bindie Mackenzie (also apparently sometimes called The Betrayal of Bindie Mackenzie), and Finding Cassie Crazy.