I think the book was written in Fairytale land. The writers said in the interview they did the other day, that as far as Mary Margaret remembers, she has always had that book. That indicates it was there from the start of the curse. I think the most likely candidates are Rumple himself or Blue Fairy- Blue Fairy has known since the beginning that Rumple was working to create the curse. Maybe she wanted to make sure all the stories of her world were recorded before the curse struck.
I don’t think Belle is the author, but it’s not implausible so who knows?
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