You may need to include a "DOI"/"doi" for some styles.  DOI stands for digital object identifier, and if the book or article does not have an online equivalent it is unlikely to have this identifier attached to it. The DOI system began to be applied to online materials in 2000. However some materials published previously to 2000 have had DOIs applied to them retrospectively.  DOIs are standard, so there would not be more than one for any article or book.

Books may have a DOI number if the publisher has requested one but most print and ebooks are not assigned a DOI.

• Book DOIs need to be registered by the publisher
• ISBNs and DOIs are not the same thing, although a DOI may be constructed from the ISBN (Called an ISBN-A). This is done by the registration system.
• In most citation styles, a DOI is not required for a correct book citation.

Scholarly journal articles will generally have a DOI number. Usually this is included somewhere in the citation or found somewhere on the article at the top or bottom of the page.

• If an article/chapter from a book is available separately online, or if it appeared in another source (such as a journal) previous to being included in the book, then it might itself have a DOI.

Newspaper and Magazine articles generally do NOT have a DOI number assigned. You may try CrossRef query to see if there is one but generally these types of sources are not given a DOI.