By Yuki Urushibara. Released in Japan by Kodansha, serialized in the magazine Afternoon. Released in North America by Del Rey.
I will admit, I always enjoy a volume of Mushishi when it comes out. Just like most folk stories, it has a certain relaxed, ambling pace that is suited to the scenic, watercolor art. Even in black and white, the art still looks like it's being painted rather than drawn. Ginko is a good POV character (calling him the hero seems wrong for this series), and the whole thing is well-written.
And yet, after reading it, I rarely go back to reread. I feel no desire to watch Mushishi AMVs, or read Mushishi fanfics. In fact, I'm not sure there's much of a Mushishi fandom at all. This is no Naruto, or even a Honey & Clover. It's a work of art, but you feel like you're seeing it in a museum. It never comes home and sits above your fireplace.
I had mentioned folktales, and really, that's probably the best way to describe the plot of Mushishi. As Ginko wanders from village to village, he comes across a new vaguely supernatural phenomenon, all of which lead back to the Mushi that are part of the title. Sometimes Ginko is able to solve things and leave everyone happy, and other times the solution brings only sadness, or at least melancholia. And then he moves on. There's no regular cast, which is probably just as well as the one-shot characters who are not Ginko tend to look very similar from volume to volume.
If there were any other genre I would ascribe to Mushishi, it would be horror. Not the gore-filled shock horror of many other manga titles, or even the corpse-laden sarcasm of Kurosagi Corpse Delivery Service. This is the sort of horror you get after waking up from a long nap to discover that weight on your chest is the corpse of a little girl. Disquiet wrapped in a blanket. Several times in this series I've debated putting it down, as I don't really like horror, but it never quite hits the level of doom and destruction that other series do.
The strength of the series, and possibly also its weakness, is its slice-of-life mentality. Here is humanity, laid out for you as Ginko goes around the country. There are cruel people, and loving people, but mostly we see a lot of people trying to get by and distrustful of anything that's different. The Mushi may not be an evil thing when it comes down to it, but their effects - and what they do to the lives of the people that come on contact with them - are chaotic and frequently undeserved. But then, life goes on, and is rarely fair.
Mushishi takes place in all seasons, and some of the stories are in blazing heat, but for some reason I always think of it as being a winter manga. Cool, yet not frozen, Mushishi trudges along, looking gorgeous and inspiring much thought. But it's also almost impossible to get close to. This is a series that you can write essays about, but will never hug to your chest and go "Squee!".