The core idea of modernism is the lack of system and pluralism of perspective. This is why you wont finde methods. You find many books analysing music of these composers, however. The music is not centered on harmony anymore. The tonal unity is broken in order to give espace to other paramaters. This is why only a holistic approach gives the basis to understand these composers.
In some cases we can even say that only aesthetics can give the basis for the analytic discurse, like in case of Wolfang Rihm’s Deus Passus and Jagden und Formen
However, to understand well impressionism and expressionism, so much as neo-classicism, it is REQUIRED that you understand well the common practice at least in form and harmony. By expressionism i mean even the serial atonality.
In the picture you see a very curious way of Ligetti using triads.It is important to know tonal harmony in order to understand how and what to avoid.
