Give them all number 0?
Is it really important for you to know that the book is part of the Barnes & Noble Classic Series?
I would add that information to a tag. Not use any specific field.
Oh. Before you buy more, check if the same book is available for free from here. You might also want to compare the ones you've already bought with the books available here. The books from here might look better and be smaller, besides being free...
And most likely the metadata is fine, without any annoying superfluous marketing blurbs.