I think the 'User Defined Fields' feature (already in pipeline) will solve this and a lot of other similar requests.
Meanwhile, the best option would be using tags or comments with the caveat that we will not be able to sort on that information.
Creating two records of the same book is a good idea but I would not personally do it as it will give a different book count and keeping other metadata in sync could be an issue.
My 2c worth
