Originally Posted by Jellby
Maybe opening several instances of Sigil, each with a different ePUB file, and drag&drop between them?
I don't know--but I admit I wish to high heaven I could do this; I have authors who have multiple books, and I'm constantly re-creating the same content (e.g." About the Author" and, more importantly "other books by this author" replete with thumbnails that have external css styling [float, padding and such], etc.), and although I copy the text, natch, it's a royal PITA to reinvent the wheel each time.
EDIT: I used the "unzip the old file and upload the existing items" trick and it was spiffy. Thank you, Valloric, for turning me on to this, it's saved me no end of brain-damage.