Gravity Falls has a new Bill Cipher book for adults - Polygon

Gravity Falls’ big villain, Bill Cipher, is getting his own book written by series creator, Alex Hirsch. And it’s for older readers! Out in July.

Bill Cipher, Gravity Falls Wiki

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A Code to DeCipher (BillDip) - Chapter 1 - Wattpad

The Book of Bill, Gravity Falls Wiki

Gravity Falls / Funny - TV Tropes

Alex Hirsch

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Why Gravity Falls: The Book of Bill Has A SECRET Version

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