I have an experimental new version of EpubMerge I could use some volunteers to test.
While EpubSplit is the natural opposite of EpubMerge, because of the way EpubSplit works, it doesn't give you back quite what you started with in terms of interior structure. Plus you have to manually select where to split for each merged book to get them back.
This EpubMerge version offers an alternative though two new options.
First, there's a Keep UnMerge Metadata? option in the configuration for EpubMerge now. If set, a copy of the original metadata for each merged book will be included, allowing for UnMerge. This includes your calibre custom columns. Leave off if you plan to distribute the epub to others.
Second, there's now a menu option under EpubMerge to UnMerge Epub. You must have Keep UnMerge Metadata? when you merge the epub before you can use UnMerge.
(There's also a menu now under the button with Merge, UnMerge and Config.)
UnMerge will extract all the original source epubs in (I believe) very nearly the same form as they appeared originally into new books in calibre. If you already have the same books in your library, you will be offered the option to add another, overwrite the epub in the existing book, or discard the extracted epub.
I've interested in hearing if a) this seems like a useful feature and b) if it works as expected for you.
Update Feb 19 2013 -- Remove obsolete beta versions
Last edited by JimmXinu; 02-19-2013 at 12:14 PM.
Reason: Remove obsolete beta versions