Writing Documentation Using DocBook: A Crash Course is a nice tutorial that should quickly get you “up to speed” with the basic DocBook tags. (The section on installing and using the Linux RPMs may not apply to you.)
DocBook: The Definitive Guide Homepage is the website for the book which is available in its entirety online. The book and website provide the complete (and did I say definitive) reference for all the DocBook tags. The online version of the book covers version 4.3 of DocBook. The print version covers an earlier version of DocBook.
The website for Sagehill Enterprises has an HTML version of DocBook XSL: The Complete Guide which is an essential reference if you intend to do any serious publishing with DocBook XML. There is also a PDF version that can be purchased and downloaded.
DocBook Open Repository is the SourceForge project where the DocBook Stylesheets and other tools are maintained.
DocBook FAQ provides a large set of questions and answers about DocBook markup, tools, stylesheets, etc.
DocBookmarks is a large collection of DocBook links.