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/DigitalIdentifiers_07Jan08.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.