DOI
A digital object identifier (DOI) is both a unique identifier (like the ISBN) and a new kind of permanent hyperlink on the Internet.
As an identifier it provides some of the same benefits as the ISBN — in fact the ISBN itself can be used as the suffix of the DOI, especially where the DOI is used for a book at the title level.
But the benefits of the DOI extend beyond the physical book at the title level — e.g. it can include all the different formats of the book (print, audio, HTML, PDF, e-Book) and also individual chapters, illustrations or other “components.”
It can also facilitate supply-chain interactions in the online environment because of its dual role as both identifier and actual hyperlink.