I gather that you want the range of volume numbers for a set and not a book? Can you be sure that with a set, there will be no gaps in that range of volume numbers?
A calculation field in Set might be defined as:
SERIES_VOLUME::volumeNumber & " - " & Last ( SERIES_VOLUME::volumeNumber )
You may need to specify a sorted relationship for Series_volume to get the correct values for that and this assumes you want the range for the set and that you do not have gaps in that range.
I wasn't able to get that to work, I am not sure if it is because there can be multiple series for each book. Through lots of trial and error, I was able to get the following to work:
In a calculation field in SET:
Min ( BOOK::calc_seriesVol ) & "-" & Max (BOOK::calc_seriesVol )
In a calculation field in Book:
It seems like I should be able to do this more simply, but it seems to be working. I am worried it may slow things down as the DB gets bigger, but for now it will do. I simplified the set up a bit, because actually, the set and book are instances of the same table, so that throws another semi-confusing issue into things. I am still not sure that is the best way to go, but that is another question!
Sounds like you did not sort the relationship as I indicted might be necessary. That said, if your approach works, there's no compelling reason to change it.