For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Jack wakes up drunk in the city, and realizes he needs to get out, and fast. He makes his way to Monsanto's cabin.
Act II
Jack returns to Big Sur two more times before the novel's climax. In the meantime he meets up with his old friends, (Cody included), and begins a messy love affair with Billie. Act II ends at the peak of Jack's madness, Chapter Thirty-Seven.
Act III
Jack recovers from his madness by simply falling asleep. He ends his novel on a surprisingly optimistic note, though whether or not anything has been resolved is subject to debate.