Originally Posted by theducks
I suppose you could convert the books to EPUB, then ADD them together in Sigil.
Not automated, but gives you a chance to 'touch up' things as you go along.
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.