DocBook: The Definitive Guide
Norman Walsh and Leonard Muellner
O'Reilly & Associates, Inc.
1st Edition October 1999
ISBN: 1-56592-580-7
DocBook XSL: The Complete Guide
Bob Stayton
Sagehill Enterprises
3rd Edition February 2005
DocBook resources at docbook.sf.net. (XSL Stylesheets, Customization Layers, etc.)