Another area I know for printing on demand has to do with databases that are constantly being updated, and sorted different ways. The one I know of is connect with the Metropolitan Opera Archives, though I'm sure there are others.
They issued a big 2-volume edition in 1985, covering their first 100 seasons -- a fat day-by-day listing, and then opera totals for the participants. But of course it was out of date by the time it appeared, plus there are ways to look at the information that it doesn't provide (who-all has sung Aida for instance, from most to least frequent).
So they are now offering on-demand services, so that you can order the Eleanor Steber Book, and your bound volume will have the full info for each performance she sang, plus different listings of her role totals etc. Or anybody else you want, or (I believe) other opera or role compendia.
I wonder if they'll ever publish a full updated edition for general sale.