if this is your first art history class, i would start out with sister wendy beckett’s excellent books. there are other more standard textbooks but they are INCREDIBLY boring and you will probably be using one of them in your AP class anyway. sister wendy beckett is really great at looking at works of art and teaching her readers how to understand art. i would go for her most comprehensive book, The Story of Painting. It’s a great read and will get you excited about your AP class. Good Luck!