Silo This! Siloing issue with KW targets and multiple categories

[DownPour]

I am having a difficult time determining how to silo the content for this website (douwnpour).

The issue I am having is that as I see it there are several different top-level keyword targets to put at the top of the silos, however due to the nature of the products they fit in almost every one of the top-level categories.

For instance our main keyword term is "Audio Books" (and derivatives thereof). but we also want to target "Audiobook Downloads" and "Books on CD".  Due to the nature of the products, almost every product would fit in all 3 categories.

It gets even worse when you consider normal book taxonomy.  The normal breakdown would be from audiobooks>Fiction(or Nonfiction).  Now each product also belongs to one of these categories, as well as "download", "CD", and "Audiobook".

And still worse, our navigation menus link every page on the site back to all of these categories (except audiobooks, as we don't really have a landing page for that besides the home page, which is lacking in optimized content, but is linked from every page on the site.)

So, I am finding siloing, or developing a cross-linking plan that makes sense very difficult.  It's much easier at the lower levels, but at the top things become muddy.  Throw in the idea that we may eventually get e-books as well, and it gets even muddier.

I have some ideas of how to deal with some of this, such as having the site navigation put in an i frame, instituting basic breadcrumbs, and building landing pages, but I'm open to any advice or ideas that might help, especially with the top level taxonomy structure.

TIA!