Anyone know this book?

There's this book I read in like fourth grade. It was about a girl that finds out that an author is writing these books and controlling the whole town with whoever reads them. I think one of the books is about gummy worms and everyone starts eating gummy worms. Well eventually the girl goes to the author's mansion and gets to the bottom of it but I can't remember how the book ends. If you could help me get the title of this book, I'd really appreciate it!