[enuddleyarbl]

SOLVED: To make a copy of a book, select the source book in the library, go to Add Books and select (oddly) "Add Empty Book (Book Entry with No Formats)." On the resulting dialog, enter the information you want and, down at the bottom, select "Also copy book formats when duplicating book." Hit the "Duplicate Current Book" button. That does it.

OP: I guess I've lost my mind. I want to split an epub3 compendium into its component books. EpubSplit works only with epub2. I know I did this once before where somehow I told Calibre to copy a book, then I edited each copy to delete the stuff I didn't want. I'm pretty sure it was at the Calibre Library level. I swear it asked me something like which formats to copy and how many copies I wanted. But, I can't find that function any more. Any ideas?