Aside from just admiring his work, I own two of his books where he explains all the symbolism and thought that he puts into each and every painting. He purposefully offsets things from the center to draw the person's eye. Sometimes he'll rely upon geometric shapes (like triangles) to add stability to a painting (having a protagonists legs spread while they stand creating a triangle in your mind. He talks about creating chaos in a painting by just shifting the horizontal line of the ground up and at an angle. Anyway, I found it all fascinating.
Michael Whelan is old now and he rarely if ever does work for publishers because he doesn't need to. He's rich, successful, and in the twilight of his career. I know that he recently did a cover for one of the new Robert Jordan books and the artwork is so good it makes me want to read the whole series.
How important do you think cover art is to a book?