Rough sketches incorporating problems involving design, light, skies, water, trees, shadows, color, and space that are frequently encountered by amateurs are juxtaposed with reproductions of finished oil paintings demonstrating solutions
There were a lot of before and after cases, and good painting principles were applied. Even though it was published in 1976, it had a lot to say to painters at all levels of development.
One of the best art how to books I've seen. Full of before and after photos of paintings and lots of ideas for making landscapes interesting - including light, color, shadow, adjusting shape. My favorite is when they point out that you can just move things around a bit to make a better composition, I don't know why that's such a revelation.