Contents
The problem with DocBook is not a lack of documentation. In fact, there are probably too many guides for getting started that well meaning people have posted on the web over the years, but you may have difficulty figuring out which of these guides is current, which addresses your needs, and so on.
The official DocBook website.
The DocBook Open Repository at Sourceforge
From here you can download the latest release of the stylesheets. The docs page includes a list of resources. Start with Five steps for finding answers to DocBook questions.
The docbook and docbook-apps mailing lists
Bob Stayton's Using the DocBook XSL stylesheets
DocBook, The Definitive Guide: The online version is more up to date.
Notice that you have to go to two separate places to get the DTD and stylesheets. That may seem strange (and even inconvenient), but there's a reason behind it. The DTD dictates the semantics of a DocBook document. There are some processing expectations associated with many elements, but it is not a requirement and further, there is no presupposition about what tool will be used to process the document. The stylesheets maintained at the DocBook Open Repository are not the last word on processing DocBook documents. You are free to build or buy a different implementation because you own your data.