The official reference documentation for
DocBook is DocBook: The Definitive
Guide by Norman Walsh and Leonard
Muellner. It is published by O'Reilly & Associates, Inc.
and is also available online here.
The current online version is 1.0.3.
A Note on Book Version
The version number of the online book
can be understood as follows: the first
number is the edition of the printed book,
the second number is the reprint number,
and the third number is the online
publishing number. So, version 1.2.3 is the
third online edition of the second printing
of the first edition of the book.
You can read the book online or
download it in several forms:
The official homepage for DocBook: The
Definitive Guide is http://docbook.org/.