I think that one of the best art books ever written is, Understanding Comics by Scott McLeod. Don't let the title fool you.
It's written as a graphic novel and takes a single sitting to read.
The reason I think it's important is that it has less to do with comics and more to do with building a large context that encompasses all art. This is done visually by using a scale from photography toward abstraction and eventually to hieroglyphics and and then to writing. A second dimension is then added to gauge design and then a third axis to gauge dimension. His form allows the placement of anything considered art within it, and in reference to any other object under the heading of "art"....., high, low, European, African, cave paintings..... ,all of it.
What this does for me, is to take the piss out of , "this art form vs. that art form", that wastes so much time. It is then, only a matter of good or bad. There ends up being both good and bad contemporary art, modern art, 19th century art.... That's worth arguing about. It's not whether "an installation isn't good because it's not a painting" or " painting is a higher art form than drawing" or "a graphic novel can't be art." etc.
McLeod also states that there are only 4 ways to make art. - art about the world around you. - art about your inner life, - art about art, or -art based on someone else's ideas.
Most used bookstores have it for 5-10 bucks.
[abebooks.com]