Getting Started with SGML/XML
http://www.oreilly.com/catalog/docbook/chapter/book/ch01.html#AEN958

Walsh, Norman and Leonard Muellner
O'Reilly and Associates
1999

Abstract:

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.