Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"For madmen only"
What's up with the epigraph?
The epigraph is a line from the book itself, and it comes from the sign over the disappearing door advertising the magic theater. It also shows up when Harry finally gets invited into the Magic Theater, which must mean that he has finally become mad enough to get into the crazy club. So part of letting go of the Steppenwolf has to do with accepting his own madness and letting his freak flag fly.
We could also read the epigraph as a warning to readers of Steppenwolf, because the book itself might be only for madmen. In that case we, just like Harry, are about to discover a whole new way of thinking as we join him in the Magic Theater. Are you ready to open your mind?