Originally Posted by Valloric
On the other hand, merging epub file contents is something you can do easily with Sigil today. Let's say you have epub A open in Sigil, and you want to merge it with the contents of epub B. Simply unzip epub B and in the Book Browser select "Add existing items" and select all the XHTML files from epub B. This will alsopick up all the CSS and image files that those XHTMLs reference. The epub B XHTMLs will be appended after the epub A XHTMLs in the Text folder.
Presto, contents merged.
The issue is that you need to do prepare the second ePub file (unzipping it) outside of Sigil rather than from within Sigil. How about if you select "Add existing items" and then select an existing ePub file, it would display the contents of the file. Then, when you select a specific file from within the zipped ePub, it would unizp just that file and insert it into the ePub file you're working on.
(And, I suppose, ask you to rename the inserted file if it has the same name as an existing file within the ePub file.)
The point would be to do as much within Sigil as possible, rather than requiring preprocessing by the user.