The format is similar to Eyewitness Guidebooks, in that the information is presented visually with lots of sidebars and with a storytelling style of writing, great for a general survey. She has included an illustrated time line for each major period of art history, which is great for visual learners.
Sister Wendy tells the history of painting in terms of the constant sweep towards and away from Classicism, the swinging between Northern and Southern European influences, and between Catholic and Protestant perspectives. After reading the book I feel like I could plot all of art history along these major axis.
As I read the book I put a sticky note on every painting I found especially interesting, and now I'm going back through all the bookmarked pages to do sketches. I've found when sketching from a heavy book, it's good to set it up in a cookbook stand. And try to keep the cat away from the piles of graphite shavings.