I have a number of books from SFBC, most do not have ISBN. Some people put SFBC after the publisher name ie. Doubleday SFBC , ACE SFBC. While that lets you know its SFBC there is no way to sort on that to get a list of the books. I wonder if it would be better to list that as an Edition on the Details tab? Also a number of books in core have ISBN and are listed as SFBC, where are people finding that (Yes newer issues do have the ISBN)?
If you want a list of those books I would recommend putting something in the Tags field. You can later click View > Folders > and group by the Tags field to quickly find them then.
As most Book Club releases are later releases, I’ve created custom editions to cover this, BCE, and choose this as the Edition in the drop down list.
Dont know if that helps you.
I decide to do it this way (edition field) also, Tags would work but with multiple tags its harder to find. I don’t put other editions in except First and I use the radio button box for it. I’m still trying to decide how to handle the SFBC numbers, currently using a user defined field. AJ suggested Tags for it but that would be a new tag for each number resulting 200+ tags, most are unique. I thought about the number line but worry if I uploaded to core, it would confuse people.
Numberline is not a field in Core, so feel free to use that field if you like 
Thx