Another good source of information is -photography- books. They deal in lighting and lighting theory, composition, camera placement, and so forth. The higher end books can deal with the kinds of make up you would need to get a certain effect, which can be applied in a paint program. CG exists orthangonally to traditional 2D media on material art, photography, and cinematography. None of those disciplines addresses the reality, but each one has handy things to steal and apply. And like they've all said. Practice. Or more accurately, Experiment. High res textures can be a good resource...but if your lighting isn't right, the render will still look like the bottom of the birdcage. That's the fun part of this whole hobby; despite naysayers and genre snobs, there -is- no 'make art' button in any of these programs. They and the resources you can buy or download are nothing but tools. What you make with them, that is true art.