So one of my favorite landscape painters are Edgar Payne. He does have a book called "Composition of Outdoor Painting", a book I have heard amazing things about from some people, while other people describe it as unreadable. At some point I'll probably try to get hold of a copy, but for now I think I'll just enjoy his paintings. I have wanted to do some studies for a while, and the last two weeks I pulled myself together and got around to a quick-study every morning.
I think you can learn so much about shapes from this painter. Very helpful to paint through what I imagine to be his process.