Originally Posted by JGB
and also do programs like epubcheck help me verify the created epub files?
That is in fact what epubcheck does.
You can edit an EPUB book using 'unzip', the text/HTML editor of your choice, then 'zip'. Check out HXA's "Epub Format Construction Guide"
for some details on the necessary arguments to the 'zip' command-line tool when re-archiving the book. (You're on your own if doing this with a GUI -- best of luck.)