The Lost Mind - Christopher Pike, D. Barbour Awesome. Body-switching! Amnesia! Drugs! Astral projection or whatever!

Jennifer wakes up next to a dead body, but she can't remember what happened, who she is, or anything at all. She slowly realizes that the dead girl was her best friend. All the evidence points to Jennifer as the killer, but she doesn't remember, so she reads her own diary to try and piece together her life.

I always loved that image, of having to figure out your personality by reading your own diary. Would you like yourself? Would you think you're shallow? Oh Christopher Pike, you posed many philosophical questions to my middle-school self.