Understanding Comics first attempts to define what really makes a comic, i.e., when we step back from genre, label, etc., what really makes a comic?
Second, he dares to ask "what is art" and "can comics be art".
By establishing the roots of comics not in the current century, or even the 1800's, but rather in the pictograms of the Egyptians and further back, Scott McCloud states clearly how he sees comics as an art form.
And then he wrote Reinventing Comics...