Landscape photographers really need to also understand what's above the land: the sky. One of the best books I've found on that is John Naylor's Out of the Blue (affiliate link). Day, night, weather patterns, how light works, it's all here. Galen Rowell was the one that turned me onto using scientific examination of phenomena to better my photography, and I'm pretty sure he would have loved a book like this.
I still like Joe McNally's The Moment it Clicks (affiliate link). I don't include this in the composition category for a reason: I don't think you can take away any specific composition advice. Instead, you get the story behind the photos, which is fascinating in and of itself, and sometimes one of the details in the story makes something click in your mind, too. Entertaining, thought-provoking, and sometimes providing a useful tidbit. Can't top that, especially when Joe does it with his usual larger than life style.