This is a very good—verging on excellent—read about Caspar David Friedrich. I think it shows some of the best formal analysis writing I have ever seen, breaking down the mechanics of how a painting works (and supplementing this with iconographic, biographical, etc. components, but honestly we're really here for the formal analysis.) It should not be the first, or only, thing you read about this artist, but it makes a good addition to other treatments of his work.
I did have some issues with the book: I think the organization makes little to no sense, and I think a stronger grounding in Friedrich's biography would have helped with this. I wish Koerner had outlined his decisions about which paintings to discuss, and I wish there had been more explicit takeaways about what the analysis of each painting tells us about Friedrich's approach to landscapes.