I'm rarely intimidated by a textbook but I was intimidated by a set of cardiology books I saw at the vet school. The topological structure of waves in the heart is

https://en.wikipedia.org/wiki/Spiral_wave

because cardiac cells circle around a loop with phase from 0 to 2π, contrast that the usual oscillator which has position p and momentum q. An oscillation in that space can rotate around the center and look like a phase but it's also possible to go right through the center, whereas for cardiac excitations the p variable is on the unit circle. This astonishing book covers the topology of this kind of thing:

https://archive.org/details/geometryofbiolog0000winf/page/n9...

particularly the cases where you have just one phase (e.g. jet lag or cell division or plant phenology) but it applies as well to those spiral waves where every element in the medium has a phase.