If you looking for an online service that will let you write and publish your own ebook in a format that is compatible with most ebook readers (including Sony Reader, Nook and Amazon’s Kindle), check out Adobe Buzzword at the new Acrobat.com.
Adobe Buzzword, if you are new, is an online word processor (like Google Docs but "more beautiful") that you may use to create and edit documents in any web browser. The service supports collaborative editing so multiple authors can work simultaneously on the same document from their respective location.
Now if you are looking to write an ebook in the browser, Buzzword can be a great choice as the service can export documents directly as EPUB files, a format that is supported by all popular ebook readers including apps like Stanza that let you read ebook on a mobile. Simply compose a document inside Buzzword, export it as an ePUB file and then import it in your ebook reader.
Other than creating new ebooks from scratch, you may also use the Buzzword application to convert your existing Word (.doc and .doc), or Open Office (.odt) documents into the ePUB format without requiring any desktop applications.