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

Getting Started with SGML/XML

This chapter is intended to provide a quick introduction to structured markup (SGML and XML). If you're already familiar with SGML or XML, you only need to skim this chapter. To work with DocBook, you need to understand a few basic concepts of structured editing in general, and DocBook, in particular. That's covered here. You also need some concrete experience with the way a DocBook document is structured.

Walsh, Norman and Leonard Muellner. O'Reilly and Associates (1999). Articles>Documentation>Standards>XML