Here's why it's important:
* Symbol of Gatsby's Obsession: The book represents Gatsby's desire for a simpler, romanticized past. He associates it with his childhood and his connection to Daisy. It's a symbol of his longing for the unattainable.
* Irony of Gatsby's Character: Gatsby's obsession with Daisy and the past is contrasted with the idealized image of the book. Gatsby sees himself as a hero, but the reality is he's a man deeply flawed, driven by a fantasy.
* Contrast with Gatsby's Reality: The book's simplicity and innocence are a stark contrast to the dark and complex world Gatsby inhabits. He tries to recreate this idyllic past, but ultimately fails.
* Nick's Perspective: Nick is initially fascinated by Gatsby's mysterious past, but as the story unfolds, he sees through Gatsby's facade. The book serves as a reminder of the difference between Gatsby's self-constructed image and the reality.
* Theme of Illusion vs. Reality: The book highlights the theme of illusion versus reality that permeates the entire novel. Gatsby's obsession with Daisy and the past is ultimately a false hope, and the book symbolizes the illusion of his dreams.
Ultimately, the "Hopalong Cassidy" book is a symbol of Gatsby's yearning for a simpler, happier time, a yearning that ultimately leads to his tragic downfall.