I've notice quite a few albums released also having UPC/EAN numbers also have ISBNs as well.

Structually, we are looking at a 10 or 13 character string. Spaces and dashes should be stripped out for storage. The only valid characters are [0-9X], and the 'X' should be forced into a capital letter.

Like with the UPC/EAN field, there should also be a checkbox to force include an invalid ISBN ("as written on box").

Unresolved questions: Do we have one ISBN field or allow 2 (or more) for ISBN-10 and ISBN-13? Or should we generalize a serial number field and allow it to store any number of catalogue schemes (UPC, EAN, ISBN, etc)?