To stay on topic, the OP:
August claims the Author is trapped in the book. How did the Author get stuck in there? How do they (heroes or villains) let him out?
-We already know the sorcerer trapped the author behind the door. The mechanics of how this was done is yet to be revealed.
-Perhaps Emma, the savior, needs to use the book or August’s typewriter to add to the author’s story and write him out of the door. If she does it when she is dark, then the author is released but is compelled by some kind of silly magical rule to let the villains win.
(I have already suggested that the sorcerer might be controlling what the author is writing for his own benefit, but maybe the author has some artistic liscence and he spent many hundreds of years moving chess pieces such as Rumple around to create Emma so that he can eventually be released).
Theory based on: nothing. Just a wild guess.
Keeper of Regina's bravery