For those who don't know, Danto is a philosopher and art critic who has written about the philosophy of art, among other things (see Louis Menand's piece in the current New Yorker). In a nutshell, Danto thinks Warhol was a revolutionary artist because he threw into question the definition of art more sharply than previous artists (such as Duchamp) had. If Warhol's Brillo Boxes, which consists of stacks of Brillo boxes, is art, the definition of art must involve something extra-visual or non-visual, since there is no significant difference between Warhol's Brillo Boxes and Brillo boxes that could have been found on any grocery store shelf. "What makes something art must accordingly be invisible to the eye" (Danto, Andy Warhol, p.65).
Similarly, Warhol's 1964 movie Empire, consisting of "an uninterrupted view" of the Empire State Building and running for "just over eight hours" (p.77), throws into question the definition of a movie. Empire "showed...that in a moving picture, nothing in the picture has to move" (p.79).
Danto's book also contains some humor. This R-rated passage (pp.76-77) is an example:
"In none of the silent, so-called minimalist films is there anything much to see, not even in the 1964 Blow Job, which shows the face of an attractive if anonymous young man who is being fellated off-screen. So the title seems like false or at least misleading advertising. It [i.e., the film] was too long, however short a time it lasted, and nearly caused a riot when shown at Columbia University...in 1966. The students were impatient and filled the air with boos, hisses, and jokey singing of 'He shall never come.' ... Andy was in the audience, planning to say a few words after the screening, but he left quietly when the furor started."