Product Search Engine Design with Sphinx and Faceted Search

I have never used Thinking Sphinx or even plain Sphinx, still, here are
my 2¢:

It seems to me that your problem is that you have a meta-model toolkit
on the one hand, your Category, Product, AttributeType, and Attribute
classes. While on the other hand, you want to define concrete facets
such as author, brand, etc. on top of that. That may be possible, but
probably it is not.

I've had a quick look at the code for Thinking Sphinx and you'll
probably be able to find what you need if you start studying it closely
starting from its ActiveRecord integration. I don't think you will be
able to achieve your goal without understanding details of how Thinking
Sphinx works, as you're trying to stretch it beyond its intended use.