Citations are fairly simple. Good doc writers use them liberally to avoid plagiarism and give credit where credit is due. Here's an example:
<citation><xref linkend="docbook"></citation> or <citation> <ulink url="http://www.docbook.org/tdg/html/docbook.html"> DocBook: The Definitive Guide </ulink> </citation>
And here's what it looks like formatted (Note: I left out the first one because I don't have a cross reference to it in this doc, I'm only showing the second one.)
[ DocBook: The Definitive Guide ]