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 SGML theology, and dense thickets of abstract terminology. See XML in a Nutshell [Harold-Means] for a good book-length general introduction.

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 Documents Using DocBook. This is an excellent tutorial.

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

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