Beyond: A Ghost Story by Graham McNamee

Jane is not your typical teen. She and her best friend Lexi call themselves the Creep Sisters. Only Lexi knows why Jane is different from anyone else: Her own shadow seems to pull her into near-fatal accidents. Jane is determined to find out why these terrifying things happen, and to overcome her shadow enemy. Her sleuthing with Lexi connects her own horrors to the secret history of a serial killer.