How about Maus?
I've read Maus, and Watchmen, and plenty of other graphic novels and comic collections like Cerberus, etc.
It depends on the library. A school library doesn't need to spend $ on Watchmen or Cerberus or The Dark Knight Returns or 300, anymore than it needs to spend $ getting a copy of X-Men #1 by Claremont and Lee. Some of those are really really good stories. Some of them are works of art, visually and story-wise. But they have next to zero educational or research value, IMO. Half of them are already movies anyways. I don't even like Captain Underpants or Nate the Great books in school libraries, so I'm kind of biased. I like graphic novels. I've read the best. Maus is a different story. It is a sort of cartoonish way of telling real history. It belongs in a school library because it describes an historic event. 300 does not, despite somewhat describing a historic event, because it tells it poorly, in a propagandish way.
When it comes to a public library, if they want to put in Watchmen or The Dark Knight Returns, that is up to them. The public should get what the public wants because they are the ones paying for it.
My take on graphic novels is that they stimulate different thought centers, but are equally valid as works of art.
Yes, I have no problem declaring them works of art. But not all works of art have educational value. Fine Art in it's pure form has no utility other than entertainment and bringing pleasure. I can imagine an art school wanting some graphic novels in their library, but otherwise I don't think that Superman:Red Son has any educational value. This may lead to the question, how does any fiction have educational value? Why does Romeo & Juliet have educational value and Watchmen doesn't? Why does
To Kill A Mockingbird have educational value, but
God Loves, Man Kills does not?
I have no clear answer to this. I would have to think about it. Perhaps the difference is age, or perhaps the difference is in the amount of fantasy involved in superhero books. Of course I would not argue against Tolkien, Lewis, Howard, or Martin being in a school's library. So there may be some inconsistency with my position. But I'm pretty stead-fast.