Graphic novel recommendations?

[MuckerFish]

During the day I have a couple of breaks to fill. Recently a thread was describing Arkham Asylum, and it caught my eye. I picked it up and thought it was pretty good. I followed it up with Dark Knight Returns which I thought was great. I found that having a graphic novel in my bag was a great way to fill in those couple of downtimes I have during the day, so I'm hoping for some recommendations. I'd love to pick up a series that spans a couple of books, and preferably with a similar dark feel to it like the above mentioned ones. Any recs from those more familiar with graphic novels?

Thanks -