This way of publishing might be peculiar to Germany. In any event I've never noticed it in the English speaking world.
One of my sources is a two volume publication by Jürgen Ritter entitled Garnison-Kirchenbuch Hannover 1816-1867.
It is also referred to as a "Sonderveröffentlichung", i.e. "special publication", the 28th volume of this sort published by genealogical society, Niedersächsischer Landesverein für Familienkunde. It seems that this society has a series of special publications like this.
At the same time it's also referred to as volume 125 of the series Deutsche Ortssippenbücher, Reihe B.
These two volumes can be found by just searching for the author's name and the title of the works. Should I also add these other two identifying pieces of information, just for the sake of completeness? And if so, how? Maybe just in a note with the first citation? Or does this also need to be part of the bibliography list?