Not really spoilers, more of a theory, but just in case it can spoil the story for some, I'll wrap the tags around it.
! The author is either Grunkle Stan (because he had that 6 fingered Glove and knows a lot), his twin (from the theory that he has one) or Old Man McGucket. The last one most likely because of a apparently leaked image of him writing the Journals. Also, his right hand has a casket over it, probably hiding an injury where he lost his 6th finger, and the Shapeshifter said that he (the author) hasn't been himself in over 30 years, which could explain why Old Man McGucket seems so crazy, something happened (failed experiment?) Anyway, that's what I think, but you can never be too sure in Gravity Falls.