The DOI is not in fact an identifier of digital objects. It’s a digital identifier of objects – DOIs can be assigned to physical items. But they are most frequently assigned to digitally-published journal articles.
The DOI is, like the ISBN, not a dumb number. There is a prefix and a suffix, separated by a slash. Most of the time, the prefix begins with the number “10”, followed by a period – this indicates that the identifier, while part of the Handle system, is specifically a DOI. After the period, there’s a number indicating who registered the DOI (similar to a publisher prefix). Following the slash, the actual identifier of the article can be alphanumeric.
The main DOI registration agency is CrossRef, but there is also one for the entertainment industry called EIDR. Most book publishers who are using DOIs register them with CrossRef, however - these are largely academic and scientific publishers who also publish journals.
More information about DOI can be found here.
The ISTC began development in the early 2000s as a way of collocating editions of textual works. It’s not technically a “Work ID” for books, but was misperceived that way – in actuality, it’s an identifier of text strings. So (broadly speaking) the hardcover, paperback and ebook edition of a book would all receive the same ISTC, but the French translation would not, because the text strings are different.
The ISTC is not necessarily assigned by a publisher, or a library, or a bookseller. It can be, but it doesn’t have to be. There is no “ownership” of the ISTC like there is with other identifiers.Anybody who wants to register a textual work – whether it’s an author, an agent, a publisher, or whoever - must submit a request to an ISTC registration agency with the necessary metadata needed to distinguish that work from others. The registration agency determines whether or not that request qualifies for a new or an existing ISTC.
More information about ISTC can be found here.
The ISAN was developed in the year 2000 and published by ISO in 2002. It identifies an audiovisual work – so it applies to films, TV shows, video games, etc. ISANs are used by film and television studios, services such as iTunes and HBO, and technology companies like Microsoft.
The first twelve numbers of the ISAN form the “root” of the identifier. The root is assigned to the core work. The next set of numbers applies to the episode or part (if there is one – if not, the next four numbers are zeroes). The next character is a check character. The next eight numbers identify the version of the work. The last number is also a checksum.