Okay a brief collection of links and reference to Sigil which allows epub metadata changes:
Epub on wiki http://en.wikipedia.org/wiki/EPUB
Metadata specification (epub 3.0) http://idpf.org/epub/30/spec/epub30-publications.html#sec-metadata-elem
Open source app (Sigil) that allows metadata changes http://code.google.com/p/sigil/
Link to Sigil source code http://code.google.com/p/sigil/downloads/list
If you install Sigil and open any epub book you can hit F8 key to edit meta data. There is a button that says "add basic" which gives you a list of the common epub fields used.
Basically epub is just a zipped up hierarchy of files one of which is content.opf, within here is a metadata xml field that contains all the metadata (look at it within sigil for the structure)
(As an aside using cbxshell I'm able to view epub thumbnails (after cloning the shellex .cbr registry data field to .epub) in Opus which is kinda cool.)