For me, going through wikipedia article on phong and working out the math with my friends (instead of just blindly translating the equations to code) really helped me understand the basics.

In order to learn more (I admittedly know very little about anything other than the phong model - the other friend is much more knowledgable), I presume either a textbook or a course notes like this https://www.student.cs.uwaterloo.ca/~cs488/notes.pdf will probably help.