ISBN-13 The ISBN-13 property is designed for the 13-digit ISBN number used in the publishing trade to identify books. The main functional attribute of this field is that it makes it easier to search reliably on these numbers. For example, if you enter the ISBN with hyphens in it, as is often done, the end user can enter the number without hyphens and still find the desired number. Also, the end user can do a search with hyphens, and they will find the ISBN, even if the publisher hasn't entered it that way.