First, congratulations on knowing that you have to pick 'the one best category' for your site.|
The right category for your site depends on 1) what you qualify for, and 2) where you think people might browse the directory to look for you.
People who search the directory might also find your listing by keywords. Of course, good editors remove keyword-stuffing and puffery from site descriptions, so be tasteful.
In the Shopping category, the clarity and security of the purchase process are reviewed as part of the site. Please see FAQ 3 for more information.
In Digital Books, it's important to have a 'legit' unique downloadable book, not one that's free to re-title and re-sell. That said, the quality of the book is _not_ what is being evaluated. This makes sense because the books are often not available for free at a Shopping site.
Free samples are read. Reviews from respectable sites are noted. But in the end, this is an evaluation of your site, not your book.
In the Subject categories, sites need to have enough free quality content in that Subject to qualify. If the site is 'just selling a downloadable book' it will be transferred by that category's editor back to this Shopping category for review.