https://www.amazon.com/OpenGL-Shading-Language-Cookbook-high...
https://www.amazon.de/Graphics-Shaders-Practice-Michael-Bail...
https://www.amazon.de/Real-Time-Rendering-Fourth-Tomas-Akeni...
https://www.amazon.de/Physically-Based-Rendering-Theory-Impl...
Or grab some online content instead,
http://www.pbr-book.org/
https://developer.nvidia.com/gpugems/GPUGems/gpugems_pref01....
https://developer.nvidia.com/gpugems/GPUGems2/gpugems2_insid...
https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_pref0...
https://developer.download.nvidia.com/CgTutorial/cg_tutorial...
Naturally most of the stuff I linked to, doesn't use necessarily GLSL ES, however it is relatively easy to translate the concepts and ideas into it, given how most shading languages happen to be either a C or C++ based dialect.
https://www.amazon.com/OpenGL-Shading-Language-Cookbook-high...
https://www.amazon.de/Graphics-Shaders-Practice-Michael-Bail...
https://www.amazon.de/Real-Time-Rendering-Fourth-Tomas-Akeni...
https://www.amazon.de/Physically-Based-Rendering-Theory-Impl...
Or grab some online content instead,
http://www.pbr-book.org/
https://developer.nvidia.com/gpugems/GPUGems/gpugems_pref01....
https://developer.nvidia.com/gpugems/GPUGems2/gpugems2_insid...
https://developer.nvidia.com/gpugems/GPUGems3/gpugems3_pref0...
https://developer.download.nvidia.com/CgTutorial/cg_tutorial...
Naturally most of the stuff I linked to, doesn't use necessarily GLSL ES, however it is relatively easy to translate the concepts and ideas into it, given how most shading languages happen to be either a C or C++ based dialect.