Cooper could be considered the dean of watercolor, embracing the medium totally and teaching and writing about it over the years. Taught at The Pratt Institute and was President Of The American Watercolor Society (a medal in his name is now given by the AWS). Wrote Painting With Watercolor, Flower Painting In Watercolor, Watercolor By Design, and The Art of Drapery: Styles and Techniques for Artists. He illustrated a number of books and worked extensively with Collier's, American Weekly, and Cosmopolitan.