My Best Friend's Exorcism - Grady Hendrix

Abby and Gretchen have been best friends since fifth grade, when they bonded over a shared love of E.T., roller-skating parties, and scratch-and-sniff stickers. But when they arrive at high school, things change. Gretchen begins to actdifferent. And as the strange coincidences and bizarre behavior start to pile up, Abby realizes there's only one possible explanation: Gretchen, her favourite person in the world, has a demon living inside her. And Abby is not about to let anyone or anything come between her and her best friend. With help from some unlikely allies, Abby embarks on a quest to save Gretchen.