I assume you have seen the thread
.epub format. Your best bet may be MishaS's oeb2epub.py (post #10). If you have a Windows PC, one way to get an OEB document (HTML, images and a .opf metafile) is MobiPocket Creator. HarryT's
Tutorial - How to Create a MobiPocket eBook builds an OEB using MobiPocket Creator on the way to MOBI. It is relatively easy to convert OEB to EPUB, but not all EPUB features are available in OEB.