I'm looking for a name of this book I read during Middle School I believe about a girl living in a book...?

I never finished it because I lost the book! All I remember is in the beginning, when I started to read the first chapter, it started out as the girl (main character) and the other characters realizing that the book is opening and that their characters should get into character for the reader. It seems they were aware that they were in a book and were always awaiting for the reader to come back and continue reading. I also remember reading about this girl running through the pages to get back to where she was, where the reader left off... because when the book is closed, all the characters go back to living their lives, talking, hanging out, etc. until the reader gets back. I thought it was very interesting, and whenever I would read it, I felt as if I was holding a tiny, little world in a book, in my hands...

If anyone remembers the name of this book, please do tell me. I would really like to continue reading it...

Thank you very much in advance :]