In English it's released as Mushishi: The Next Passage or The Next Chapter. No idea whether either is a exact translation.
Aside from the subtitle and a new opening theme, it's the shame show, just more stories from the manga. As far as I know, Mushishi is about as faithful to its manga as anime can get, which seems to be a very good thing in this case.
Early this year there was a new special that was double the length of a normal episode called Mushishi: Hihamukage. This was maybe my favorite of all the Mushishi stories. At the end where there would be a preview of the next episode there was an ad for the new series. So far there have been 10 episodes of Zoku Shou, and another special I haven't gotten to yet which is apparently episode 11 and 12 of Zoku Shou, and a second season is going to air in a month or two.
On a side note, what did you think of Welcome to the NHK?
That series affected me more personally and deeply than any other anime and perhaps any other media, because I've struggled with (comparatively minor) agoraphobia, paranoia, and depression. I've never seen a better take on those issues, and the show also managed to be hilarious and uplifting despite the heavy themes.