OASIS DocBook home page (also mailing lists):
http://www.oasis-open.org/docbook/
docbook-apps mailing list: http://lists.oasis-open.org/archives/docbook-apps/
"DocBook:The Definitive Guide" home page:
Includes documentation in DocBook Wiki
DocBook Open Repository:
Books:
DocBook:The Definitive Guide, by Norman Walsh and Leonard Muellner, published by O'Reilly & Associates, October 1999 (new version available online)