Heya!
Likely some people have seen this, but I stumbled across a newish GLSL tutorial / guide / book that is really nicely paced and seems to do a really methodical job of building up understanding of fragment shaders. It seems like he is still adding chapters which is cool. Also plenty of references to other great resources are mentioned within.
[url]http://patriciogonzalezvivo.com/2015/thebookofshaders/[/url]