Jtornhill has published an impressive 200 page guide to creating procedural nodes in Cycles, focusing on how to 'dirty up' your materials. The document is open to commenting too, so you can ask questions and/or help improve it!
The nodes for cycles could be a jungle for many people. I have taken it as my mission to clear things out on how you approach the nodes when creating materials in Blender.
So, for all of you that want to know the ABC on wear and tear using cycles node I have written documentation on how you could apply wear and tear using Cycles nodes. The document ends with an in depth practice and the files for those are reachable and free to use as wished.