On teleread in a comment to Cybook Opus now available: Another example of eBabel limitations
Michael Dahan says:
For US and UK customers, Bookeen has teamed up with BooksOnBoard (the largest independant american eBook retailer) to offer the best user experience. In other words, the Cybook Opus possessor beneficiates from a dedicated customer service which will help him finding its way with eBooks download. Check at http://www.booksonboard.com/opus (site is in final tweaking stage)
This site only shows ePub and PDF ebooks, and so might be useful to owners of other devices with Adobe Digital Editions. Note that visiting the site probably sets a cookie restricting searches to ePub/PDF. When I later visited the BoB home page, my searches were still Opus specific.