Certainly harmonics are common and well understood. What is different about the harmonics theory is the consideration given to harmonics of harmonics (and so on). I have not seen this addressed elsewhere. In things like guitars you naturally get harmonics, but you do not get harmonics of harmonics. That is because the harmonics do not themselves act as seeds for more waves. AFAIK the condition for this to happen is to have standing waves in a non-linear medium in at least 2 dimensions. Then all standing waves, including the harmonics, have centres where there is greater energy concentration and that is the necessary condition for developing further harmonics because the non-linearity is then greater there.