Choker - Elizabeth Woods

Sixteen-year-old Cara Lange has always been a loner, even more so since she moved away from her best and only friend, Zoe, a few years ago. Cara mostly spends her time avoiding the popular girls who call her 'choker' after a humiliating incident in the cafeteria, and watching Ethan Gray from a distance, wishing he would finally notice her.
Then one day Cara comes home and finds Zoe waiting for her. Zoe's on the run from problems at home, and Cara agrees to help her hide. With Zoe back, Cara's life changes overnight. Zoe gives her a new look and new confidence, and before she can blink, Cara is flirting with Ethan and getting invited to parties. And best of all, she has her best friend to confide in again.
But just as quickly as Cara's life came together, it starts to unravel. A girl goes missing in her town, and everyone is a suspect ~ including Ethan. Worse still, Zoe starts behaving very strangely, and Cara begins to wonder what exactly her friend does all day when she's at school. You're supposed to be able to trust your best friend no matter what, but what if she turns into a total stranger?