Originally Posted by dwig
If you take this approach you should edit the individual EPUB files prior to attempting to combine them. The major issue will be the CSS files.
The best approach is probably to use Sigil to rename the CSS files in each of the individual EPUBs so that they all have unique names. That way when the individual HTML components of the EPUBs are merged they can reference their own CSS files (which will also need to be added to the mega-EPUB) avoiding conflicts.
Actually, you need do no CSS work.
Sigil creates a separate CSS for each book imported (you can't turn it off,even if all books use the exact same CSS) renaming possible conflicts with existing CSS.
All you get is a slightly bigger file. (you can edit the reference in the book back to a common CSS if appropriate, and delete the extra