Try
Watchmen--it's one of two graphic novels I know, but it's really, really good, the sort of thing that demonstrates comics' potential to be genuinely literary. It does for superhero comics what
Blood Meridian did for the Western, if that makes any sense...(review here:
http://www.grovel.org.uk/reviews/watchm01/watchm01.htm)
The other graphic novel I know is the ten-volume Sandman series, which is also very good:
http://www.neilgaiman.com/comics/essay_sandman.asp