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Hampshire Nanny - AFAIK - There is no 'official' way to link books in a calibre library. There are roundabout ways but . . .
You could make use of
Quickview.
If the omnibus and empty books had a common value in a custom column e.g. 'Omnibus/#omnibus', then if you highlight an Omnibus cell and press Q you would have
sub-list of books in that Omnibus.
Added : Try it with a something like publisher - highlight a publisher cell and press 'Q'.
BR