Adult graphic novels are a burgeoning industry. There are a variety of choices, as any librarian or patron who has either looked at a catalog or set foot in a comic book store knows. Often, these works are looked down upon as "not really reading" but they can be a tremendous boon to your patrons in several ways.
First, if you are keeping up with the news in 2013, libraries everywhere are lamenting the lack of 20-somethings in the library. This is one of the perfect audiences for these types of materials. If you are serious about attracting this demographic, invest in some quality graphic novels.
Second, graphic novels are great for reluctant adult readers, particularly late teen and early 20s males. They can help literacy and reading skills by allowing the reader to intuit from the drawings what they may not understand from the text.
Third, readers may find a new appreciation of stories that they did not find to their tastes before. For example, many of the classics like Jayne Eyre and The Odyssey are available in graphic novel form. Many classics are.
Finally, a surprising thing about graphic novels is their appeal to English as a Second Language patrons. Again, focusing on adult literacy - those patrons trying to learn and understand English are able to intuit from the drawings what they cannot understand from the language.
In conclusion, a word of caution. Adult graphic novels are just that. For adults. Their content can be extremely...well, graphic...for lack of a better word. They should be housed with the adult materials so as not to confuse children and young teens about what they are. Often, the adult stories include graphic violence, nudity and adult situations. That is not to say that they should not be purchased, just that the selector needs to be aware of the content and plan accordingly.
On a personal note, I am a huge fan of graphic novels as I was of comic books as a child. There is much to love about the graphic novel format. Read a few of the classics and let me know if you agree. I think you may be pleasantly surprised at what you find.