There is information about the Materials System in the SL Wiki and Forum. The part I found interesting is the link to: Brief Considerations About Materials. This is an article by a teacher for a Post-Graduate Course on Game Art.

The emphasis in this article is on Specular Maps. This is something many of us have little knowledge about. But, the specular map is going to be far more complex than the normal map. This is the area where knowledge will be key to making nice object textures or great object textures.