Just some general comments:
I agree that it would be nice to be able to search by format, since I greatly prefer ePub over pdf.
Some of the book description pages didn't code correctly. Several of the ones that I looked at had html markup on them. Others were walls of text that were hard to make out on the screen. I looked some of those books up in Amazon and found the same description with better formatting.
It would be really nice if you had one entry for a book with all of the formats available, especially for the more prolific authors. I noticed with Anne McCaffrey, for instance, that one book might show up several times, sometimes in pdf and other times in ePub. It would be much easier if I could choose the book and then click on one of the formats to purchase that format.
It becomes a little tedious going through an author's works when there are several copies of the same title scattered throughout. See also above.
It would be nice if the author searches were a little more precise. I wouldn't have expected that searching on Connie Willis would bring up every Connie in your database. It appeared that most of the actual Connie Willis books were near the top, but I spotted a few further in. I don't think people will have the patience for this.
Some of the books are missing covers on the search display page and had a generic MyEZRead cover instead. Clicking on other covers opened the descriptions page, but with these books you could only click the More Info button.
It would be easier to search if there were two search bars, one for author and one for title. Yours seems to default to title and I often search on author, so that became an annoyance.
I'm impressed that it handles misspelled words well and produces titles even though they aren't exact matches. I do wish that the search remained visible in the search bar though, so that I could adjust it if I didn't get quite what I expected.
The prices are great and the search engine is very fast.
I'm not sure how you do it, but I would recommend getting your bookstore included in searches by
www.inkmesh.com and
http://ebooks.addall.com. Both of them are great places to comparison shop for books. Your prices are competitive, but people still have to find you.
Hope this is helpful.