There are many online retailers offerring EPUB format ebooks
How to publish EPUB format books?
EPUB format is a standards-compliant zipped container with .epub extension. The standards are maintained by the International Digital Publishing Forum (IDPF) at http://www.idpf.org .
ePubNow! is an online production platform for producing perfect EPUB format ebooks. However, you need to convert your book in to an XHTML file. Publishers having XML workflow can easily integrate their digital production with ePubNow!
ePubNow! also offers an automatic creation of an Online Catalog for all EPUB titles produced using its advanced technology platform.
The Catalog can be accessed by the publishers.
The Catalog + ePubNow! Online Reading System can be made accessible to the users on the Internet, and the various business models could be devised and integrated in the ePubNow! framework to monetize the delivery of copyrighted content on the Internet through a secured access system.