Lucy didn't rewrite the book, she didn't "rewrote the book in the mind", she didn't decipher the actual book, all she knew was that there was magic in it, and the magic can properly can be dispelled by other magic, but I guess the condition that the author put on the magic is that his son can automatically dispel any magic that was cast on it.

If you have read a particular author's books for a long time, and then you see something's off with the book you probably know something happened. AND NO YOU WOULDN'T JUST TRANSLATE/DECODE the book just because you're a writer yourself, unless you're THE AUTHOR, which Lucy is not in that case. EVEN in real life its impossible.

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