Gravity Falls: Lost Legends

Gravity Falls: Lost Legends is the first official Gravity Falls comic with new original content. It was announced at D23 on July 14, 2017, at Alex Hirsch and Daron Nefcy's panel promoting Gravity Falls: Journal 3 and Star and Marco's Guide to Mastering Every Dimension, respectively. It will be released on July 24, 2018.

Trivia

 * It was originally listed as XQWLWOHG PBVWHUB ERRN (which decodes to UNTITLED MYSTERY BOOK using the Caesar cipher) on Amazon Canada.
 * The front cover was revealed via a jigsaw puzzle, made of Dipper Pine Trees, piece by piece on Twitter, Tumblr, and Facebook.