I found these quite helpful when starting out with OpenGL ES, just to see what approach one would take when dealing with ES as opposed to normal GL.
http://www.zeuscmd.com/tutorials/opengles/index.php
As has been mentioned earlier there are some samples available from the iPhone developer site as well:
- https://developer.apple.com/documentation/opengles
- https://developer.apple.com/library/archive/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/Introduction/Introduction.html