My Best Friend's Exorcism (Used Paperback) - Grady Hendrix
The year is 1988. High school sophomores Abby and Gretchen have been best friends since fourth grade. But after an evening of skinny-dipping goes disastrously wrong, Gretchen begins to act ... different. She's moody. She's irritable. And bizarre incidents keep happening whenever she's nearby.
Abby's investigation leads her to some startling discoveries — and by the time their story reaches its terrifying conclusion, the fate of Abby and Gretchen will be determined by a single question: Is their friendship powerful enough to beat the devil?