My daughter found my copy of Cathy's Book and when I explained it had an ARG aspect to it she was intrigued. We called the number to see if it was still active, it is btw, and she is basically begging to read it. She is only 8 years old and I remember it being pretty mature. I told her when she is older I'd let her read it. But is there anything like it for younger readers. She reads at around a 5th grade level.
For those that don't know Cathy's Book is about a girl whose boyfriend breaks up with her she basically stalks him for answers and discovers his family secret and now they are wanted by a secret society. (From what I remember without major spoilers) In the book there are phone numbers, websites, photos, etc. in the pocket attached to the front cover. There also is drawings throughout the book due to it being a journal of a young artist. Hope this helps find something similar!
by Both-Leadership783