OpenGL
Learn OpenGL
Learn OpenGL is the definitive resource for learning real-time renderer techniques as beginners. Even though it is an OpenGL tutorial, it also teaches rendering techniques at the same time.
Best Practices for Modern OpenGL
A guide about avoiding common programmer errors by using modern OpenGL (4.5+)
docs.gl
An improvement of the official OpenGL documentation
3D Graphics Rendering Cookbook
A book that covers and compares both the OpenGL and Vulkan APIs, while also covering various recipes about making a renderer.