Though brief it tells you all you need to know and if you haven't yet please read The Book of Shaders. IMHO glsl is not beginner friendly as a whole so don't look for tutorials as you need a comprehensive understanding of it's protocols first.
There is no one size fits all with shading languages and any 20min video will only further confuse you on a very broad topic.
These aren't in the public domain but you can have a look at this or this for reference.
Amazon has some great books whose names escapes me at the moment but something to get your feet wet here