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.