Understanding Comics, by Scott McCloud [link]
I've suffered through more Art History classes than I care to recall. They taught me a great deal about what's been done before, but the great questions of why does art exist was largely left to our speculation. Likewise, the practical question of why one would choose one art style over another was always attributed purely to the philosophies of the artists through time, reacting to their environment and the work of the artists before them. This did not imbue me with a great deal of respect for art.
I really wish my Art History teacher had assigned this book. Scott McCloud talks more clearly about what art is, what it's good for, and how it functions effectively than the three semesters of lecture I had about what's gone before. It seems like practitioners of Art History talk about art almost exclusively through writing, but writing is a visual, emotional medium, and you lose a lot approaching it exclusively through text. By writing comics about art, McCloud integrates show and tell in a way that seems like this is how art was always meant to be discussed.
Anyone learning about art or design should prioritize reading this book. Suddenly, art makes sense!