The word "NOTAN" was new to me. It is Japanese meaning "dark, light." According to Dow, "Notan-beauty means the harmony resulting from the combination of dark and light spaces - whether colored or not - whether in buildings, in pictures, or in nature."
Combined with line and color, these are the building blocks to any successful composition. Harmonious relationships between the three are key. Of course Dow was writing a book directed towards art students, but the same principles, IMO, should apply to landscape photographers.