Ah, yes, Knut Hamsun, the Norwegian Nobel Laureate who eventually went crazy and gave his medal to Goebbels as a token of affection, but not before inventing the unreliable narrator, among other things.
This book is, I'm told, one of the first real attempts at an interior, psychological first-person narrator. As such there's something of a conflict (appropriate given Hamsun's combativeness) between language that's accessible, almost plain at times, and a more classical sense of diction and objectivity. But rather than take an amateur's guess at where specifically the crazed, unnamed writer protagonist fits into the evolution of narration, I'm interested a definitely prescient assumption Hamsun seems to make. Note the title: not "madness" but "hunger." This early psychologically attuned monologue treats consciousness as a physical phenomenon — mental states are portrayed as almost biochemical ones. The narrator's behavior is altered by how much food he has in his stomach; by the weather; by the sights and sounds and people he encounters. Rather than a fixed consciousness interacting with an outside world, he's a semipermeable vessel.
For what it's worth, recent editions of this book have a Paul Auster forward that initially seems overwritten, but ends up making good points about the narrator as a harbinger of Modernism — in the literary and indeed social sense — who divests himself of connections to social conventions, religion, and ultimately objective reality.