Spherical harmonics appear in several computer graphics techniques. I feel that in order to be a better computer graphics developer, I need to have a deep understanding of what they are and how they are used.
It seems that the reference most often recommended to understand Spherical Harmonics is "Stupid Spherical Harmonics Tricks" by Peter-Pike Sloan.
I started reading it but did not find a "satisfying" definition of SH, it seems like the document mostly relies on other references for the "basics". Other references introduce the Fourier Basis functions as a "simpler version" of SH, but once again it seems hard to find good material explaining them.
What are thorough, accessible references to understand Fourier basis functions and Spherical Harmonics ?