Where can I go to learn about filters? I'm getting a matte box and I want to get 4x4 glass filters to use with it --- but what types and from what vendor?

Specifically, I don't really understand things like graduated or gradual filters or what effect the different colors would have.

Is there a definitive book or other source for this information?

Thanks, Ed

Take a look at the Tiffen Filters site:
http://www.tiffen.com/Header_page_tiffen_filters.htm

You should find all you need to know there...

Robin