As indicated previously I personally believe that simplicity is best for many reasons, so I typically apply KISS.
Ex. "3rd in the Moon Walking series." The "Moon Rise" volume or group of the "Planet Series."
Also you can group within a series or across series and not necessarily in printing order. Once you decide on what the group is, then the next task is to name the group in a way that will be easily remembered.
I think the point is to give the customer a quick easy way to understand what could be complicated. Groupings should occur as the author sets forth a relationship for the story and eventually the reader.