BISG and BIC have published a discussion paper called "The identification of digital book content" - https: //web. archive. org/web/20090920075334/http: //www. bisg. org/docs/DigitalIdentifiers₀7Jan08. pdf. The paper discusses ISBN, ISTC and DOI amongst other things and makes a series of recommendations which basically say to consider applying DOI, ISBN and ISTC to digital book content.
Ed Pentz (Mon,) studied this question.