Well, we know now that Rumple remembers and he seems to want to break the curse and seems to have arranged to bring Henry and through him Emma here. So I can imagine that he wrote the book to reveal the truth to someone (and Henry is the ideal recipient) and left it to be found. I don’t want Rumple to be behind absolutely everything either but I do think that’s a plausible theory that fits a character who takes an active role in getting what he wants.
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