Docbook-xml mode is an early implementation of a Major Mode for Docbook XML written by FrancisLitterio?.
Docbookxml project page - Located at Sourceforge and characteristically inactive.
docbook-xml-mode.el - Latest version in web CVS.
DocbookIDE – Major mode for editing Docbook SGML/XML. Has been VaporWare? since 2000.
It’s worth taking a look into PsgmlMode.
And why not NxmlMode which comes with Docbook support?