True Blue by Deborah Ellis
You know how the female praying mantis bites the head off of the male? That was one of Casey's favorite things. As a future entomologist, she adored insects. She even copied the head chomp with a little hand signal. The signal meant that someone was really getting on your nerves, and you'd really love to just stop them in their tracks. That was before the murder trial.
True Blue, by Deborah Ellis, follows the arrest of high school senior Casey White from the point of view of her best friend Jess. The two girls have been inseparable for most of their lives, and Casey was planning on spending the next year studying insects in Australia.
Our tale shifts between Jess in the present and her diary entries from the month of the murder. Jess and Casey were both counselors at the same summer camp in charge of the most obnoxious camper of all time. Still, no matter how annoying the girl was, Casey wound not have murdered her....would she?
In the present, Jess betrays her friend in exchange for popularity. At first her actions seem like slip-ups, but as time passes, the mystery of Casey starts to pale in comparison to the bizarre behavior of her "best friend." We as readers find ourselves in a troubling spot, since our sole source of the story appears to have issues of her own.
Not many books for teens fall into the genre of psychological thriller, but True Blue definitely has the chops to be a riveting, disturbing page-turner.