1. Landscape Painting by John Carlson. Not an easy read but of huge importance to landscape painters. Contains critical concepts, such as the different planes in the landscape that affect value, and why. If I had read this book first I would have been convinced that I could never learn to paint, but it is a must read. All the photos are in black and white, great for studying value, but look up his paintings online so you can see them in color too.
2. Composition of Outdoor Painting by Edgar Payne. Excellent discussions regarding good compositions and bad ones, and what makes them so. If you are not familiar with Payne's work look him up and see how fabulous he was. There is a reasonably priced "coffee table" book with beautiful color photos and an extensive text called EDGAR PAYNE the Scenic Journey that gives a wonderful overview of his work and is well worth owning.
3. The Art Spirit by Robert Henri. I confess that I have never read this book cover to cover, but it contains many instructional lectures and also inspirational thoughts about painting.
I am now working my way through a series of books by Emil Gruppe. (There is an accent on the last "e") He was a student of Carlson's and famous, among other things, for his wonderful boats and trees. He actually discusses the behavior and character of trees. He walks you through many of his compositions in detail, inviting you to eliminate specific elements by covering them with your finger to see how important they are to the painting. You can find these on used book websites and the prices can vary, but they are worth the investment. Imagine a whole book on brushwork! I'm not there yet but looking forward to it.