What is a DOI?
DOI stands for Digital Object Identifier. A DOI is a unique alphanumeric string that is assigned to a source. Look for the Digital Object Identifier (DOI) when searching for electronic database articles and ebooks. Be sure to include the DOI at the end of your citation.
DOI Guidelines from APA (p. 298-300):
DOI Format (p. 299)
Why doesn't my source have a DOI?
The DOI system was created in 2000. Works published before 2000 do not have a DOI unless the publisher retroactively assigned them. Sometimes even recently published works may not have a DOI either. Here at JU, most print books do not have a DOI. An electronic book from a library database (EBSCO, ProQuest, etc.) may or may not have a DOI listed. APA does not require you to include the URL for electronic articles or books accessed from a database (p. 299). However, most professors at JU will require you to include the permalink to the source in the database where you retrieved the cited work. Librarians recommend adding the permalink to the end of the citation. Learn more about how to create permanent links. (pp. 296, 298-299)