If we take into account that Regina does in fact know who she is/what she did (Its up for debate on if she does or not). Reality seems to believe (This isn’t dealing with the show at all) that adults seem to forget/not believe in fairy tales as children do. Back to the show, Regina could be in a situation where she thinks this book is just fantasy (When we know it is true) and isn’t really concerned with it.
Now, going back to the “Regina knows who she really is” topic. If it hits her that “Oh crap, this book is giving away secrets that I can’t allow Henry to know about”, she probably would set out to destroy it. I know if something were to put my plans/perfect world in danger, I’d try to destroy it.
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