http://docbook.org/docbook-ng/, the DocBook NG Schemas.
http://sourceforge.net/projects/docbook/, the DocBook SourceForge project.
http://docbook.org/tdg/en/html-ng/, a special edition of DocBook: The Definitive Guide showing the DTD and (flattened) RELAX NG content models.
http://www.oasis-open.org/committees/docbook/, the DocBook Technical Committee.
http://norman.walsh.name/threads/refactorDocBook, a thread through the essays I’ve written about redesigning DocBook.