The music writer Donald Tovey called it "the greatest set of variations ever written" and pianist Alfred Brendel has described it as "the greatest of all piano works". It also comprises, in the words of Hans von Bülow, "a microcosm of Beethoven's art". In "Beethoven: The Last Decade 1817–1827", Martin Cooper writes, "The variety of treatment is almost without parallel, so that the work represents a book of advanced studies in Beethoven's manner of expression and his use of the keyboard, as well as a monumental work in its own right". In his "Structural Functions of Harmony", Arnold Schoenberg writes that the "Diabelli Variations" "in respect of its harmony, deserves to be called the most adventurous work by Beethoven".