Every You, Every Me by David Levithan

Every You, Every Me is a profoundly haunting novel set in the words of Evan, an overly emotional teen.  He discovers a plethora of notes, pictures, and items in suspiciously stalkerish locations.

Stalked by eerie flashbacks of his unstable best friend and her unknown fate, he goes on a wild goose chase to find her secret, and ultimately, himself.

(Edit by Miss Jennie: The creation of this book has a unique story all its own, which really feeds into the feel of the book.  A great read!)