"DocBook: The Definitive Guide"
    by Norman Walsh and Leonard Muellner
    with contributions from Bob Stayton
    ISBN: 156592-580-7

This book is a gentle yet thorough introduction to the DocBook DTD (which is
used by, amongst others, the FreeBSD Documentation Project).  A dead-tree
edition of the book is published by O'Reilly & Associates, Inc., but the text
is freely licensed under the GNU FDL.

The current edition purports to document DocBook v4.4 with the EBNF,
HTML Forms, MathML and SVG modules.

An unexpanded edition of version 2.0.17 is also available. In this version,
content models are shown with parameter entities rather than fully expanded.

WWW: http://www.docbook.org/tdg/