OpenGL
OpenGL used to be pretty easy.
Now it is not.
That means a lot of what you see on the internet and in old books that is OpenGL is nothing like what OpenGL is today. I myself am a dinosaur. I used to do all the glBegin, glCallList stuff, and I am completely out of my domain when we start talking about vertex buffers and GLSL.
I am looking at Learning Modern 3D Graphics Programming by Jason L. McKesson
I am going to write a tutorial series on a particular game I want to write for myself at DF/Tutorial. It uses pyglet.