User blog comment:Gravityfan1931/the writer of the books/@comment-11449049-20140805235138

I believe that Old Man Mcgucket is indeed the author of the books due to these observations:

1. In the episode, Into The Bunker, when the computer-that-looks-like-a-briefcase is opened. you can see at the top: PROPERTY OF F. The only person in Gravity Falls whose name starts with an 'F', is Fiddleford H. Mcgucket, also known as Old Man Mcgucket.

2. The Author of the books has six fingers, as seen on the cover of the journals, Old Man Mcgucket has a bandage wrapped around his hand, where a sixth finger could have easily been.