Why is the book "thirteen reasons why" a mystery?

Question: Why is the book "thirteen reasons why" a mystery!?
Best Answer - Chosen by Asker:

I think because no one knew way the girl killed herself and the entire time, the boy (I forget both of their names) was listening to the tape to try and figure out why she killed herself so it was a mystery for him as well as for the reader!.Www@QuestionHome@Com