Here you will find all documents that I create about the nodes. Some of these are also translated to other languages which is more than appreciated.
One “off topic” download here as well, and that is for those that still wants to use the 2.79 theme on 2.8 :). Then you can find it here: http://www.blenderinsight.com/wp-content/uploads/2019/01/2_79.zip
Beginners guide to cycles nodes, the procedural way (English)
Procedural wear from A to Z in Blender (English)
Below is translated by Xevi Porras López, a big thanks :)!
Guia d’iniciació als nodes de cicles, de manera procedural (Catalan)
I also have an in depth tutorial on how to use the apple shader