I didn't know this existed, however, searching using some general keywords don't seem to give any hits. This includes titles which are available at stores like eBookMall, fictionwise etc.
I have got some positive responses from the contacted ebook sellers, and I have a good feeling about this. No matter how the ebook sellers change their sites, as long as the API is the same, the UESE (Unified Ebook Search Engine) should work like a charm.
It would also be very easy to add new eBook sellers to the engine.