The Book of Shaders
thebookofshaders.com
thebookofshaders.com
You can do pretty cool procedural stuff in a vertex shader (without any vertex data inputs). It's a necessity for animated models.
There's even less material on geometry and tesselation available.
This book seems to have rather nice pixel shading tricks but I'd like to see some stuff about the other stages too.
Though there is some unexpected behaviour on the site. After load, it always automatically scrolls down to the last editor window, as all editors seems to be selected on load. This is on Chrome Version 49.0.2623.110 (64-bit)
One major stumbling block with graphics is that you'll find lots of one letter names. "xyz" represents locations in the world, whereas "st" represents locations along a surface area. "uv" is also sometimes used for texture coordinates. There is a difference between them, but it's not interesting or relevant to a beginner. Just know that both are in use.
I would guess that "pct" is an abbreviation for "percent", given that it ranges from 0.0 to 1.0. It's definitely a poor choice of variable name.