This book will be based on things already mentioned in the show and other stuff that may of been missed.
Will contain spoilers as i am watching them and explaining some stuff.
Feel free to also mention things I may of missed and I'll put them in.
So as we know Dipper and Mabel stole the Time Travellers Time Machine
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And from there they travel back to fix the time loop so Dipper can win a prize for Wendy and after a few tries Dipper get Mabel to help him and she it works it messes up the time line for Mabel to win Waddles. A fight between Dipper and Mabel happens and the zapp in between different time line from the past but what we didn't notice is at this scene
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When Dipper and Mabel transport themselves to the Mystery Shack we see the Shack before it became the Mystery Shack and then we see
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Stanford open the door, how do we now this look at Stanford's nose
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And this look and Stanley's nose
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Stanley's nose has the little bumps on it and Stanford's doesn't.
So basically we seen the Author of the Journals in S1E9 and we had been trying to figure out who the Author was.