Cool, I recently started with GLSL, and it’s interesting there are many roads that lead to a similar solution (how I draw circles or make a center is tad different)
I was around and writing OpenGL when we started getting the first programmable cards. Nvidia shot out first with their Cg language, but I held back and waited for the "Orange Book" to come up. I bought it right away and learned from it and by looking at other peoples shaders.
https://duckduckgo.com/?q=opengl+orange+book
Also the quick reference card helps
https://duckduckgo.com/?q=glsl+quick+reference&t=h_&ia=web