One of the things that makes learning DocBook difficult is that the sites related to it tend to overwhelm the newbie with long lists of W3C standards, massive exercises in markup theology, and dense thickets of abstract terminology. We're going to try to avoid that here by giving you just a few selected references to look at.

Michael Smith's Take My Advice: Don't Learn XML surveys the XML world from an angle similar to this document.

Norman Walsh's DocBook: The Definitive Guide is available in print and on the web. This is indeed the definitive reference, but as an introduction or tutorial it's a disaster. Instead, read this:

Writing Documentation Using DocBook: A Crash Course. This is an excellent tutorial.

There is an excellent DocBook FAQ with a lot of material on styling HTML output. There is also a DocBook wiki.

If you're writing for the Linux Documentation Project, read the LDP Author Guide.

The best general introduction to SGML and XML that I've personally read all the way through is David Megginson's Structuring XML Documents (Prentice-Hall, ISBN: 0-13-642299-3).

For XML only, XML In A Nutshell by W. Scott Means and Elliotte “Rusty” Harold is very good.

The XML Bible looks like a pretty comprehensive reference on XML and related standards (including Formatting Objects).

Finally, the The XML Cover Pages will take you into the jungle of XML standards if you really want to go there.