I came across a new ebook listing directory called Ebookism. From the announcement:
eBookism is a new shared ebook directory, listing ebooks of any format. Submitting an ebook for inclusion in the directory is done via an email form, into which publishers enter book details, information and sales links, and upload a cover image. In return for being listed, publishers display an eBookism badge on their web sites.
It's not searchable yet, but once it is this could be really interesting.