So this is my theory (don't blame me if i'm not the first one who tought of this). It all starts with Stanford,Stanley and Mcgucket. It would make sense if these three would work together, because of the three keys that had to be turned. The machine was originaly created to create knowledge, sounds like something a proffesor/scientist would say, Mcgucket. The trio made the portal and tested it, but what if it got wrong, remember Mabel floating really close to portal. Maybe Stanley exctually flew inside it when he triend to shut down the portal with the red button. And the creatures came trough the portal, including Bill. My theory is that Stanley only wrote the first book, and Mcgucket the other two. In the end scene Stanley only picked up the first book, and his coat only had one slot for one book. We haven't seen much of the first book. Maybe Bill is only discribed in the last two or one. That would make sense because of what we've seen the first book is more about the portal and the third more about the creatures that came trough the portal. In the leaked picture you could also see mcgucket writing a book with six fingers. Maybe Stanley and Mcgucket both have/had six fingers. Maybe Stanley was planning to write more books (if you look really closely at the part with the photo of Mabel and Dipper you can see other books with nothing on them). Maybe Mcgucket and Stanley both have/had 6 fingers. Now to Stanfords part. Stanford knew about the apocalypse. They've given us a few hints about that. Stan obviosly tried to save his brother, but maybe for another reason. Troughout the season we have theorised about Stanley being smart. If he's smart enough he could maybe come up with an idea to stop Bill. That's why Stan is putting so much effort saving his brother. His making fake ID's and even pretends to be dead, just because he needs to hide from the police and steal the necassery equipment and most importantly, hide to portal. Stanley is the only one to save them, to save the universe from Bill Cypher. ( Also for the people who don't know what the credit scene is about, remember the swing in Stan's mind). Sorry for my bad English.