A couple of suggestions:
1) Add an input field for submitting a "site recommendation", ie, for telling you about a site that carries e-books that you're not currently including. For example, a search for "Daniel Keys Moran", a well-known SF author from the early 1990s, finds no results, yet all of his works are available for free (legally) from
http://immunitysec.com/resources-dkm.shtml. I imagine there are many sites like this for various authors and e-books, and your customers can be an excellent source for finding them. Obviously, you wouldn't want to add any sites without checking them out first (to avoid pirate sites, porn sites, etc, from being added).
2) Figure out some way to filter out books that are included simply because an author gave the book a "cover blurb". For example, search for "Lois McMaster Bujold" and it comes up with 898 entries. Unfortunately, she's not THAT prolific.

MANY of the books listed are by different authors entirely, and I assume they're included because a cover-blurb by Lois was included on the books. Some folks might want to search for those kinds of cover-blurbed books, but most folks won't, I imagine.
3) Include some way to group all 'editions' of the same book together. Again, looking at the Bujold listing as an example, each book is listed MANY times, and I (as a user) would like ONE link for ALL editions of a single book. What seems a little strange with the current display: there are multiple entries listed for "Sharing Knife, Volume Three, The", most of which are in the $17 to $22 range, but one was listed as $9.99 and had 2 sites. So I clicked on that one, and (as expected) the $9.99 site was Amazon, but then the other site was BooksOnBoard for $19.45. Why would BooksOnBoard be grouped with Amazon, at two wildly different prices, while others are grouped in other... er... groups? I think I know: because the 'wording' of the title is different on different sites. Some include the 'series' title only with "Volume 3", while others include the volume title of "Passage", etc. Difficult nut to crack, but hey, that's your job!
But a great start! Many wishes for expanded success!!!