Docbook XSL : the complete Guide http://www.sagehill.net/docbookxsl/index.html
The Duck Book - Docbook : the Definitive Guide
The KDE documentation primer : recommandations to write correctly in Docbook - http://i18n.kde.org/doc/doc-primer/index.html
Doxygen web site - http://www.docbook.org/tdg/en/html/part2.html