Jack's alcoholism/delirium tremens/madness
We get the sense that Jack has been under this "dark power" for some time now. And in fact, many of these Booker stages are going to have to be somewhat flexible to fit the plot of Big Sur because Kerouac did not write a classically composed novel. Still, it will be interesting to see how Big Sur fits into the Booker format.
Jack's time alone in Big Sur
According to Booker, this is the stage where "all may seem to go reasonably well." Jack writes that the solitary time in Monsanto's cabin does his soul good. There are those "signposts" of something wrong, but for the most part, our "hero" is happy.
The signposts get worse; Jack begins to disintegrate
This is where the dark power grows stronger until it imprisons the hero. In this case, Jack's worsening visions, drinking, and paranoia begin to consume his character, the novel, and even the text's prose.
This part of the novel is quite literally a "nightmare stage." Jack's delirious nightmares mark the height of his madness, paranoia, and fear. As Booker writes, "it seems that dark power has completely triumphed…"
Jack falls asleep; when he wakes up, the world is golden heaven
In a typical Booker Rebirth plot, the hero is saved by a young woman or child. This is not the case here. The "young woman" (Billie) is suicidal or possibly even filicidal (i.e., wanting to kill her child). According to our hero, the "child" (Elliot) is a "cretin." Neither of them will be saving Jack. Jack's rebirth is self-propelled, though we are left to wonder just how long this recovery will last.