Why Should I Cite?
A Digital Object Identifier is a unique number assigned to an article (not books at this time). Not all articles have DOIs. To get DOIs, a publisher has to choose to be a member of one of the DOI registration agencies, a process that requires a membership fee, and not all publishers choose to do this.
If an article has a DOI, it will usually be found on the first page of the article, in the citation, or on the publisher's page for the article. You can use the CrossRef Simple Text Query Form form to see if an article has a DOI (email registration required).