The ".epub eBooks Tutorial" provides a good overview. A sligly more polished presentation is ion "EPUB Resources and Guides" By Keith Fahlgren. Of the tools avialable, calibre looks promising as a free open source HTML or ODT (Open Office Word Processing format) to Epub ebook converter. However, I have not been able to get it to run on Linux so far.
Feedbooks provides a useful online tool for creating and publishing free ebooks. It uses a familar looking web based editor to create the book. It would be interesting to see if this could be added to online commercial book publishing systems like Lulu.com. Currently these assume you prepare the book offline and then upload the typeset result, usually as PDF. But for books which are mostly text it might be easier to provide a web based editor.