First off, Knut Hamsun's name is fun to say. Go on, try it: Ka-noot. Ka-noot, Ka-noot, Ka-noot. See what I mean?
His novel Hunger, on the other hand, is not so much fun. It's a fantastic piece of work, the kind of brilliant that makes you realize oh, now I see why he won a Nobel Prize. It just isn't a barrel of laughs, is all.
The plot is fairly straightforward: the protagonist is a struggling writer, and he starves for days at a time because his written output doesn't bring in enough money. The real character, though, is the process of starvation itself: the way it pains and gnaws at him, making him lightheaded and giddy and weak as he sobs and rants his way through increasingly desperate attempts to either fill his belly or forget his hunger for a few brief moments.
I've been told by a few people that Modernism really starts with Hamsun. After reading Hunger, I'd say that sounds about right.