I've linked to this blog in particular in relation to a school assignment for my Media Technology in Teaching class for my Master's in School Library Science.
I believe the graphic novels and manga are a legitimate source of literature. I have seen many examples in the public library as well as school libraries. Manga and graphic novels are becoming increasingly popular with teens and young adults as well as some younger elementary age students. I have even seen certain books made into a graphic novel format, such as Neil Gaiman's Graveyard Book and Ransom Riggs' Miss Peregrine's Home for Peculiar Children.