The Lonely Dead by April Henry
For seventeen-year-old Adele Vanderarde, the dead aren't really dead. She can see them and even talk to them. When Adele realizes her ex-friend Tori is really a murder victim, she takes it upon herself to help Tori find her killer - before the killer finds Adele - or before Adele is charged with the murder. Master mystery-writer April Henry weaves another edge-of-your-seat murder mystery - this time with a chilling paranormal twist.