Virtually all poetry since Wordsworth is about the poet. In the era of media, externality, and technological bombardment the inner world of the artist is one of the few places where authenticity and the quest for meaning and reality can be found.
We live in a world where all externality and all art are in many ways reduced to Trump and his tweets. Sorry if I offend anyone.
Cooke seems to have a good sense of this.
The self in its fullness is how the world can be experienced and communicated. The most revealing and moving artists are those who feel the deepest and see the farthest into themselves and become representatives of the world of human achievement. Wordsworth, Keats, Beethoven, Monet, Rilke, Proust, Whitman, Dickinson, Kandinsky Woolf, and many others.
And I would put Mahler among those poets of self and cosmos.