^There is the comic book which Joss writes. I have not read it myself.
They're not ongoing comics though. So far there has only been the 3 issue Those Left Behind
released in 2005, which takes place before the movie. And the recent early 2008 3 issue Better Days
, taking place between the end of the TV show and Those Left Behind
Both are written by Whedon and Brett Matthews, who wrote the ep "Heart of Gold"
TLB is pretty good, while BD is fairly "meh"
There are plans for a new miniseries called A Shepherd's Tale
, about Book's backstory, for release sometime in 2009