Well, a lot of what we consider to be graphic novels are technically trade paperbacks, a compilation of a storyline inside a comic series. Watchmen, The Dark Knight Returns, Batman Year One, etc were originally comics (In Watchmen's case, a limited series, and in the other two and a lot of other Batman trades, storylines that took place in the Batman series).
I am reading Watchmen right now so I'm glad I bumped into this thread, still pretty early on though. I had never heard of it before I saw the trailer but I was very impressed from that so I wanted to read it before the movie comes out. Very enjoyable so far.
Other graphic novels I'd recommend are Batman: Year One and Batman: The Long Halloween, the former is Frank Miller so definitely a good read and the latter I actually enjoyed more, it has a much more engaging story I thought. The Dark Knight Returns and The Dark Knight Strikes again are ones that I'd still like to pick up eventually.