I wrote this series of blog posts on render maps a couple of years back. Here is the first one
https://land8.com/profiles/blogs/rendering-tutorial
https://land8.com/profiles/blogs/rendering-tutorial-bump-maps
https://land8.com/profiles/blogs/rendering-tutorial-1
it goes from transparency maps to bump maps to displacement maps though ideally all three are combined in Vray.
Cgtextures and mayang are good websites and if you are using Vray and studio max vray-materials.de is really good.
Peter Guthrie’s website is a good source of tutorials.