If you are citing a book, you only need to include a DOI for an ebook because DOIs (digital object identifiers) are only assigned to digital sources. That said, not all digital sources have a DOI because this is an opt-in system. If you are using the library's databases to access ebooks, DOI information is sometimes found in the detailed description for the ebook. You can also use Crossref.org's Metadata search to see if an ebook has a DOI assigned to it. Note that unique DOIs are sometimes assigned to ebook chapters.