DocBook Wiki  

DocBook is officially available as a [WWW] Document Type Definition (DTD) for both XML and SGML. It is unofficially available in other forms as well.

Docbook.org (2003). Resources>Documentation>XML>DocBook

DocBook: The Definitive Guide  

This book is designed to be the clear, concise, normative reference to the DocBook DTD. This book is the official documentation for the DocBook DTD.

Walsh, Norman and Leonard Muellner. Docbook.org (2003). Books>Documentation>XML>DocBook