Godot uses it's own subset of GLSL, but it is effectively the same. I mean, not all the functions are ported over, and there are some differences in the built-ins, but it is essentially standard GLSL. So general OpenGL books should help you. I never actually learned OpenGL, but I did a lot of work with DirectX and HLSL is not all the different either. Once you see the logic and math and the algorithm, the slight syntax differences don't matter much.