There is a new Q&A with GRRM on nymag.com. Here's a teaser:
Some of your hard-core readers, myself included, have spent a lot of time speculating about how many different kinds of magic there are in your world. Or is it all the manifestation of the same mysterious supernatural forces?
That's something I like to reveal little by little.
I can tell you generally that when treating with magic in fantasy, you have to keep it magical. Many fantasy writers work out these detailed systems, and rules, and I think that's a mistake.
For magic to be effective in a literary sense, it has to be unknowable and strange and dangerous, with forces that can't be predicted or controlled. That makes it, I think, much more interesting and evocative. It functions as a symbol or metaphor of all the forces in the universe we don't understand and maybe never will.
Follow this link to read the full interview.