I remember reading a young adult novel, in English, in the early to mid 2000s. It was unique in that it came with a packet of papers that the main character came across during the course of the story. Ex: newspaper clippings, business cards, a torn up photograph, sketches. It even had numbers you could call and listen to voice mail. The book itself was written journal-style, with lots of doodles and crossouts and notes in the margins.
It was the journal of a girl that had gone missing, and was being read by her friend trying to find her. The missing girl was kind of a trouble child, did a lot of drawings (I distinctly remember her drawing cormorants) and ends up meeting this guy and having a thing for him. Not long after they meet, she disappears and her friend goes looking for her and finds the diary.
The cover was pretty much all black with doodles and stuff, and it had a very unhelpful title like Stacy's Book or Clara's Book or something like that. The back summary said something like, Do not read unless you are [insert friend's name here].
The reason I am asking the scifi SE is because the ending revealed that
The guy the girl fell in love with is actually from a family of immortals and he was trying to see if the missing girl could have the same genetics to be immortal as well