Eclectic and Self-Referential
The novel plays with what we think a novel should be. It's not exactly a self-contained book that begins and ends when you open and close it. Instead, it's made up of all these weird documents, like the preface that says that the novel is supposedly made up of Harry's papers:
And now that we come to these records of Haller's, these partly diseased, partly beautiful, and thoughtful fantasies, I must confess that if they had fallen into my hands by chance and if I had not known their author, I should most certainly have thrown them away in disgust. (45)
So why are we reading them? Oh, yeah, because Harry's a remarkable guy. But see how the novel is playing with us by referring to, and second-guessing, itself?
Also, Harry likes to write poems within the novel, and even remembers them and quotes them, as though they had come from somewhere outside of the novel. It's all wrapped up onto itself, which makes it hard to figure out where the fiction ends and the reality begins.
This makes Steppenwolf seem like it might apply to anyone's life, like it really is a philosophy for living and not just a novel. In fact, our reading the novel is kind of like Harry reading the treatise on the Steppenwolf—everybody has something to learn.