
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,
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.
As opposed to the red book (reference guide)[1] and the orange book (shading language reference)[2] of which I got copies of the respective last versions before they merged into the newer "red-ish book" that combines both[3].
Just in case you ever wondered where that one Lego picture in the Windows 3D maze screen saver came from (the cover of the red book).
[1] http://www.opengl-redbook.com/
[2] https://www.amazon.de/OpenGL-Shading-Language-Randi-Rost/dp/...
[3] https://www.amazon.de/OpenGL-Programming-Guide-Official-Lear...
